416 



HISTORICAL GEOLOGY. 



similar form to the preceding, but round or blunt-edged (Figs. 361, 362) ; another, the 

 Cestracionts (Fig. 357), an ancient type, of which only one genus now exists, has a pave- 

 ment (Fig. 363) of small bony jjieces (Figs. 364, 365) in the mouth (for grinding up shell- 

 fish, etc.), and a series of smaller teeth at the margin, with the mouth and cavities of the 

 nostrils confluent. Many ancient Sharks, like a few of the modern, had large spines 

 connected with, and usually along the anterior margin of, the fins (Figs. 355, 356). As 

 these fishes have the vertebral column imperfectly ossified when not cartilaginous, the 

 fossils are mostly teeth, spines, fragile vertebrae, and occasionally shagreen. 



In the lowest group, the Chimo&rids, there is a cartilaginous notochord multii licately 

 subdivided, the sheath of which is partly ossified. The species have a few very large 

 teeth, and a single gill-opening, which is covered by a fold in the skin. To this group 

 are referred the Acanthodians, which were formerly supposed to be Ganoids. They 

 have very small rhombic scales, a spine along the front margin of the fins, and are 

 apparently without teeth. 



355-365. 



Selachians. — Fig. 355, Spinax Blainvillii (x J); 356, Spine of anterior dorsal fin, natural size; 357, Ces- 

 tracion Philippi (x g) ; 358, Tooth of Lamna elegans; 359, Id. Carcharodon angustidens; 360, id. Noti- 

 dauus primigenius; 361, id. Hybodus minor; 362, id. Hyb. plicatilis; 363, Mouth of Cestracion, showing 

 pavement-teeth of lower jaw ; 364, Tooth of Acrodus minimus ; 365, id. Acrodus uobilis. 



2. Ganoids or Gars (Figs. 366 and 375). — The Ganoids have the skeleton cartilagi- 

 nous in the earlier kinds, but more or less ossified in the later and in the few modern 

 species ; one gill-opening ; a gill-cover, and gills free ; an air-bladder, having a pneu- 

 matic duct ; embryo sometimes with external gills. Skin covered commonly with 

 thick bony scales, like Reptiles or ancient Amphibians (whence ganoid, from the Greek 

 ydvo's^ shining), or with bony plates, somewhat turtle-like ; scales often rhombic and set 

 together like tile (Figs. 366, 375) ; and interlocked by projecting points (Figs. 367, 368); 

 sometimes cycloid and imbricate. Tails of ancient species vertebrated or heterocercal, like 



