It appears thus that far from being deficient in representatives of the tribe of Choudrostomi, North America has a greater number of them and more diversified ones than Europe, belonging to four distinct genera: Exoglossum, Raf., Pimephales, Raf., Cumpostoma, Agass., and Acrocheilus, Agass.; to which must be added two other new genera to be described hereafter, founded also upon North American species: Hybognathus, Agass., and Hyborhynchus, Agass. I am unable to say whether the genus Cochlognathus, B. and G., belongs to this tribe or not, as I have had no opportunity of examining it.

Acrocheilus, Agass.

The type of this genus has a general resemblance to the type of my genus Chondrostoma, inasmuch as the mouth opens transversely under the snout, and has a hard cartilaginous or rather horuy edge. But it differs from that genus in having a solid rim along the upper lip similar to that upon the lower, and in the character of its scales, which resemble more those of the group of Barbus, thau those of the common type of Leuciscus or Ca

tostomus.

As a genus I would characterise it by the peculiar structure of the edging of the mouth, which in the lower jaw constitutes a transverse broad flat plate, very similar in appearance to the dental plates of Myliobates, being thicker along the outer edge and tapering gradually along the inner edge. This transverse plate is square and cut at right angles externally towards the symphysis of the two jaws. In consequence of this peculiar structure of the margin of the mouth and its armature, the lower jaw is as it were cut transversely, and has in no degree the rounded outline about the symphysis of its branches which is observed in most Cyprinide. The membranous fold which extends from the suboperculum along the interoperculum towards the symphysis of the lower jaw is limited by a deep furrow which terminates somewhat behind the horny plate of the lower jaw. Along the inner edge of the intermaxillary bone there is a similar transverse bony plate which is, however, much narrower and rounded, folding over that of the lower jaw when the mouth is shut. Sideways and above, the intermaxillaries are surrounded by a fleshy lip which is beut forwards at the angle of the mouth to unite with the edge of the horny plate of the lower jaw. The upper inaxillary bone forms a slight projection behind the angle of the mouth in a depression arising from a membranous fold upon the sides of the lower jaw, and below and behind the first suborbital bone. There is not the slightest rudiment of a tentacle in the angle between the lower termination of the intermaxillary and upper maxillary. But what is particularly striking in the structure of this fish is the circumstance that the horny covering en

circling the mouth is deciduous, at least in specimens preserved in alcohol, showing that the attachment of this indurated edge is not very close. The surface to which the horny plate of the lower jaw was attached appears fibrous upon the removal of that plate, and the fibres run in a longitudinal direction up and down. The tissue of the plate itself is also fibrous and the fibres have the same longitudinal direction throughout its thickness, so that the plate breaks very readily at right angles with its own greater diameter.

The nostrils, two on each side as in all Cyprinidæ, consist of a tubular opening in advance and a large crescent-shaped opening

behind.

The opercular apparatus and the branchiostegal rays, present no peculiar characters. The branchiostegal membrane however unites with the skin under the chin on the anterior margin of the humerus, so that the brauchial opening does not extend to the sides of the tongue bone.

The dorsal begins opposite the insertion of the ventrals, which are themselves somewhat nearer to the anal than to the pectorals. The dorsal extends as far back as the anterior margin of the anal. It has three small rays in advance of the longest simple ray which is followed by ten branching rays, the last of which is properly a double ray.

All these rays are deeply divided longitudinally and transversely articulated. The caudal is very powerful, and remarkable for the many simple rays which it has along the base of its two lobes, there being seven above and seven below, gradually increasing, so that the longest reaches nearly half the length of the longest simple ray which edges the fin above and below. The inner rays are all deeply divided longitudinally and transversely articulated. The number of rays in the upper and lower lobe is equal, eight in both. There seems however to be a middle ray, so that properly speaking there are seven rays in the upper lobe, one in the middle, and eight in the lower lobe.

The tail is deeply furcate. When the fin is shut, the innermost rays overlap each other, so that the caudal appears much narrower than when fully expanded, but the outer rays in both Jobes remain in one plane, and do not overlap each other at all. The anal consists of two simple short rays in advance of the long simple one followed by nine articulated rays which, when the fin is closed, overlap each other; so much so that the fin appears much narrower when bent backwards than it is in reality. My attention not having been called formerly to the manner in which the fins shut until I began to study the Balistidæ, I am unable to say how far in varions families, the closing of the fins varies; but it is a point to which the attention of Zoologists should be directed in future, as it will no doubt afford interesting characters SECOND SERIES, Vol. XIX, No. 55,--Jan., 1855.

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in addition to those exhibited by the structure of the rays themselves. As far as I can ascertain, it has been admitted among ichthyologists, that the change of form in the fins arose from the rays being brought close together, or stretched asunder; I find, at least, no mention in any description, of rays overlapping each other, as I have shown it to be the case among Balistidæ, and as is also the case among many others. In the Scomberoids, for instance, the rays of the vertical fins are remarkably spreading when contrasted with those of the Balistidæ, or those of the genus Acrocheilus described here. The ventrals are rather large, somewhat similar in their rounded form and the thickness of their rays, to the ventrals of the genus Tinca; the first ray especially is thick and simple. It is followed by eight articulated rays. The pectorals are also somewhat rounded, but not so much as the ventrals, the upper angle projecting more. Their first ray is also thick and simple, and is followed by sixteen articulated rays gradually tapering, the last of which, however, are simple. In its general form, the fish upon which this new genus is founded, has considerable resemblance to the European Chondrostoma Nasus, and I should not be surprised at all if, upon a superficial examination, it had been identified with it, notwithstanding the generic and specific differences, to which I have already alluded. The scales, however, present a striking difference. They have not, as in Chondrostema, the ordinary type of Leuciscus, but resemble rather the scales of Barbus in their elongated form, their small size, their many radiating furrows diverging in every direction, and their ornamental pigment cells which are especially numerous along the posterior margin. The centre of radiation is far in advance of the centre of form. The lateral line arises above the posterior and upper angle of the operculum, and is first slightly bent downwards, so that it follows in its course upon the side, a direction nearer the abdominal margin than the back; but upon the tail it is strictly upon the middle of the side. The tubes of these scales arise in the middle of the anterior field, and taper towards the middle of the posterior field, where they terminate. The scales along the back, upon the neck, between the pectorals, and along the lower margin of the abdominal cavity, are much smaller than upon the middle of the sides. There is a naked space behind the pectorals in which the muscular swelling of the base of that fin is received, when the fin is bent backwards. There is also a narrow smooth space above the ventrals; along the base of the dorsal and anal the scales do not extend quite to the base of the rays, but upon the caudal they cover their base completely and even extend somewhat along the sides of the middle rays.

Water pores besides those of the lateral line, are very distinct upon the neck in advance of the scales. The whole surface of the skin covering the skull seems also to be perforated by a set

of smaller pores, but larger ones follow the margin of the preoperculum, and the lower jaw as well as the suborbitals and mastoid bones.

Unfortunately the two specimens collected in Columbia River are deprived of their intestines, and in one of thein ouly, were the pharyngeal bones, with their teeth, preserved; but these afford further evidence of the correctness of my view in considering these fishes as a type of a distinct genus peculiar as far as is now known to the northwest coast of America. There is but a single row of teeth and only five teeth in that one row on the left and four on the right side. The isolated teeth stand on a cylindrical peduncle swelling into an obliqne club-shaped crown, which is elongated externally into a sharp hook, but the inner surface is cut obliquely like the incisors of Rodents, and present a dat grinding surface resembling closely the dentition of Chondrostoma and Chondrochilus, differing however in the more clubshaped form of the teeth, and the sharp terminal hook, and also the smaller number of teeth in one row. Fig. 9, a, represents

9.

d

the right pharyngeal of Acrocheilus alutaceus seen from behind, b the same seen from its inner margin, c one tooth in profile from its upper side, d another from its lower side, and e the same from the inner side to show the grinding surface. As a further resemblance to the genus Choudrostoma, I should mention the circumstance, that the peritoreum is also black.

Acrocheilus alutaceus, Agass, and Pick.

Caught at Willamet Falls, and in Wallawalla River. Nose prominent and rounded.

Tail rather slender. Candal large. Dorsal much larger than the anal. The color light brown above, (there being a white and very fine line on the edge of each scale,) blending into yellowish brown upon the sides, and passing into pure white upon the abdomen. Gill-cover golden brown. Dorsal and caudal of the same color as the sides of the body. Pectorals orange, gradually paler towards the base. Ventrals as the pectorals, but more unformly orange. Anal also orange, but more bright and reddish. It occurs in the rapids and falls of the River. Is caught by the natives while fishing in the Falls for Salmon.

(To be continued.)

ART. XIII.-Thoughts on Solution and the Chemical Process ; by T. S. HUNT.

By solution, as distinguished from fusion or volatilization, we understand in chemistry the production of a homogeneous liquid by the combination of two or more bodies, one of which must itself be in a liquid state, while the others may be liquid, solid, or gaseous. The solvent action of acids and alkalies upon bodies insoluble in water is by all admitted to be chemical in its nature; but according to Leopold Gmelin, "mixtures of liquids, and solutions of solids in liquids, (as of acids, alkalies, salts, oils, etc., in water and alcohol,) are by Berzelius, Mitscherlich, Dumas, and others of the most distinguished modern chemists, regarded as not chemical unless they take place in definite proportions." "Mitscherlich attributes such unions to adhesion, Dumas to a solvent power intermediate between cohesion and (chemical) affinity, and Berzelius refers them to a modification of affinity, while proper chemical combinations according to him result not from affinity, but from electrical attraction."-(Gmelin's Handbook, English ed., vol. i, p. 34.)

The learned author of the Handbook objects to these views that "they restrict the idea of a chemical compound within too narrow limits," and he elsewhere implies that the force which produces solution is a weak degree of chemical affinity. (Id. vol., p. 70.) The judicious Turner also speaks of ordinary solutions as instances of chemical union ;* and Mr. J. J. Griffin has insisted upon the same view. As these writers have not however sufficiently dwelt upon the important principle, rejected by so many names of authority, that all solution is chemical union, we propose to offer some considerations upon aqueous solution, and endeavor to show that the process presents all the phenomena of chemical combination. First, in the fact that the resulting saturated solutions are perfectly homogeneous; secondly, in the condensation and more or less perfect identification of volume observed in the process;‡ (some anhydrous salts dissolve in water without increasing its volume.) Thirdly, in the change of temperature which attends the process; thus oil of vitriol, hydrate of potash, and many anhydrous salts evolve heat when dissolved in water, while sal-ammoniac, nitre, and many hydrous salts produce cold by their solution. Fourthly, in the change of color which attends the solution of some salts, as the chlorids of nickel, cobalt, and copper.

* Elements of Chemistry, 7th ed., p. 139.

+ L., E. and D. Phil. Mag., 3d Series, vol. xxix, p. 299.

See my paper, Considerations on the Theory of Chemical Changes, etc., this Journal [2], xv, p. 228, L., E. & D. Phil. Mag. [4], v, 526, and Pharm. Centralblatt, Leipzig, 1853, 849.