22 



COPE 



(metapterygiura) enclosed in body-wall 



and basilars only free; Acanthodii. 



Paired fins archipterygial ; unibasal ; a basi- 



occipital element; Ichihyotomi. 



Paired fins pluribasal (tribasal) ; no basioccip- 



ital element ; Selachii. 



The order Acanthodii presents the most primitive type 

 of paired fins known in the Pisces. The vertebral axis 

 is notochordal, and the fins do not display any actino- 

 trichia or rays. The males have no clas'pers. Tail hetero- 

 cercal. In the known genera the fins are supported by a 

 large spine in the anterior border of each ; and the integu- 

 ment is covered with small quadrate granules, which also 

 extend over the fins. The teeth are absent or minute, or 

 consist of a single compressed triangular cusp, with smaller 

 laterals. 



Fig. S.—Pleuracanthus lucius Ag. From the Coal Measures of Alsace. From 8auvage. 



The Acanthodidse have but one dorsal fin, while the Isch- 

 nacanthidse and Diplacanthidse have two. In the Dipla- 

 canthidse only have clavicles been observed; in the other 



