394 STUDIES FOR STUDENTS 



Class : Pisces. 



Sub-Class : Marsuftiobranchii. 

 Ostracodermi. 

 Elasmobranchii. 



Order : Selachii. 

 Batoidi. 

 Sub-Class : Holocephali. 

 Dipnoi. 



Sirenoidei. 

 Arthrodira. 



Sub-Class : Teleostomi. 



Order : Crossopterygii. 

 A ctinopterygii. 

 Sub-Order : Chondrostei. 

 Teleocephali. 



The first of these, theMarsupiobranchii, are not well understood 

 in their relations to the true fishes. The most common of the 

 group are the hagfishes and the lampreys of the present time. 

 They differ from all other vertebrates by the entire absence of 

 the lower jaw and of the pelvic and pectoral girdles of bone 

 that support the hind and the fore limbs. Whether these con- 

 ditions are the primitive stages of a developing fish or are the 

 final stages of a degenerate structure, is still an unsettled ques- 

 tion, and it is at this point of difficulty that we turn to the 

 palseontological record. However, we can gain but little from 

 the palaeontology of the forms. A single specimen from the Old 

 Red Sandstone of Scotland is the representative of the fossil Mar- 

 supiobranchii, and it is even doubtful whether this specimen is 

 correctly referred to that group. The specimen shows the pres- 

 ence of well defined rings in the position of the vertebrae, a stage 

 in advance of the recent forms, which would indicate for them 

 a degenerate structure. 



The earliest remains of fishes known are from the Lower 

 Ordivician rocks of the Grand Canyon region of the United 

 States. These are the very imperfectly preserved remains of 

 what seem to be scales and bones of fishes whose affinities can- 

 not be made out from the material. 



