28 COPE 



Subclass II.— HOLOCEPHALI. 

 But one order of this class is known : 



A single external branchial fissure ; actino- 

 trichia present; basilars, axonosts, and 

 neural spines articulating with each 

 other; pectoral fin pluribasal, with 

 three axonosts and numerous basilars ; 

 ventrals with elongate axonosts and 

 basilars; Chimseroidei. 



In all known CniMiEROiDEi the teeth are large paired 

 bodies, one pair in the lower and two in the upper jaws, 

 composed of coarse vascular dentine. These contain 

 columns varying in number and shape, consisting of coarse 

 tubes with calcareous walls which terminate on the masti- 

 cating surfaces. Notochord persistent, the ossifications, 

 when present, consisting of delicate rings, more numerous 

 than the neural arches. In the existing forms the males 

 have claspers like those of sharks. 



Chimseroidei appear in the Corniferous Limestone in Ohio, 

 Wisconsin and Iowa, and in the Eifel Limestone in Rhine 

 Prussia. They are found in Triassic and Jurassic beds in 

 Europe, and abound in the Cretacic of Europe and North 

 America, and New Zealand. They extend through the 

 Cenozoic beds of Europe and North America, and a few 

 species still exist. The known families are the Ptychodon- 

 tidse (of doubtful reference), the Squalorajidse, Myriacan- 

 thidse, and Chimseridse, the last named only still remaining 

 alive, in four genera. 



