Lign. 186. Lepidotus. Wealden. (1/6 nat. size.)
(Showing the Homocercal Tail.)
Lign. 187. Amblypterus. Carboniferous. (1/4 nat. size.)
a. The heterocercal tail.
In the Annals of Nat. Hist, for 1848, p. 304, Prof. M’Coy has described and figured an intermediate form of tail, which he regards as characteristic of the Diplopterus (of the Old Red Sandstone) and its allies: this the Professor terms the Diphycercal tail.
From this brief summary of the essential characters of those durable parts of the organization of fishes, which most frequently occur in a fossil state, we pass to the investigation of some illustrative examples of this class of organic remains. But before describing any entire specimens, it will be expedient to notice the separate fins, and teeth, which abound in many deposits; in some instances occurring in connexion with other parts of the skeleton, but more generally detached, and constituting the only evidence of the existence of numerous extinct species and genera. The greater part belong to the first order—the Placoidians (Poiss. Foss. tom. iii.), and to the families of Sharks and Rays. The osseous dorsal rays of cartilaginous fishes (named Ichthyodorulites (fossil-fish-weapons) by Dr. Buckland and Sir H. De la Beche) first demand our notice.
Ichthyodorulites. [Lign. 188].—This name is applied to the fossil spines, or rays, of dorsal fins, of which numerous species occur in the Secondary deposits; they belong, for the most part, to extinct cartilaginous fishes of the Cestracionidæ and Hybodontidæ groups. In the osseous tribes the dorsal spines have at their base two articular processes, by which they are united to the osselets that support them, as in the Silurus; but in the cartilaginous, they have no articulations at the base, and terminate in an obtuse point, which is implanted in the flesh; the posterior margin is grooved almost to the upper extremity. They are of a fibrous, osseous texture. The common Spinax, or Dog-fish (Acanthias vulgaris), has a spine of this kind in the front of each dorsal fin. The rays of the Sharks are compressed, and some have rows of teeth on the posterior margin; in the genus Cestracion (Port-Jackson Shark), these organs are strong, triangular, straight, pointed, rounded in front, flat at the posterior face, and widest at the base; in the Hays they are flattened or depressed.