PISCES. 



71 



ORDER ELASMOBRANCHII. 



The Sharks, Rays and Chimerae wliich compose this class 

 (called Selachia by Miiller, and Placoidei by Agassiz), agree in 

 general structure with the Ganoids. They differ chiefly in the 

 character of the derma-skeleton and the arrangement of the gills, 

 the former consisting of scattered bony or tooth-like grains, 

 tubercles or spines, and the latter being fixed and contained in 

 sacs or pouches communicating with the pharjaix, and externally 

 by slits. In the Chimerae there is only one external aperture. 

 Like the Dipnoans and Ganoids the Elasmobranchs combine 

 reptilian features with embryonic fish characters. 



Being Cartilaginous, they have left little behind save spines, 

 teeth and scales. The fossil spines are called Ichthyodoi'ulites, 

 and abound in the Secondary deposits. The Cestracionts began 

 in the Lower Devonian, and culminated in the Trias. The 

 Hyhodonts began in the Carboniferous and culminated in the 

 Mesozoic. The Squalodonts, or True Sharks, began in the Juras- 

 sic and culminated in the Miocene. The Bay's first appeared in 

 the Carboniferous, and ChitncBroids in the Oolite. 



No. 115. Carcharodon megalodon. 



Tooth. The huge teeth of this largest shark are found in great numbers 

 in the Marine Tertiary of the Atlantic border. Some of them are six inches 

 long and broad, and indicate sharks fifty or sixty feet long. From the Eocene 

 of Ashley River, South Carolina. 



No. 116. Lamna elegans, Agassiz. 

 Teeth. From Eocene of Ashley River, South Carolina. 



No. 117. [343] Edestes vorax, Leidy. 



Portion of Dorsal Spine (cast). This remarkable 

 ichthyodorulite was discovered in the Coal Measures of 

 Indiana, and is preserved in the Cabinet of Amherst Col- 

 lege. It is distinguished for the large proportional size of 

 the marginal teeth and their close resemblance to the jaw- 

 teeth of Carcharodon, their enameled border being finely 

 denticulate. Size, 9x3. 



