4 I 2 STUDIES FOR STUDENTS 



II. Aclinopterygian, the spine-finned Teleostomes. Fins 

 supported by dermal rays ; ancient fin support greatly reduced, 

 implanted in the body wall. Includes Chondrosteans (Ganoids) 

 and Teleocepliali (Teleosts). 



Among the fossil forms of the Crossopterygians Gyropty chins 

 and Osteolepis from the Old Red Sandstone of Scotland are very 

 similar to the early Dipnoans in the general appearance of the 

 body. The tail is somewhat heterocercal and the dorsal fin is 

 divided up into segments ; but the teeth are numerous and 

 arranged along the edges of the jaws instead of being repre- 

 rented by single plates. The entire body was covered by solid, 

 bony scales the outside of which were covered by a layer of 

 shining enamel. The anterior pair of the paired fins is 

 approaching the condition in the Dipnoans, the archipterygium. 



Holptychins, from the same locality and horizon as the last, is 

 peculiar in that even at that early date it had developed the 

 cycloidal, horny scales of the modern fishes, both the anterior 

 and posterior paired fins are archipterygial in structure and the 

 caudal fin has become nearly diphycercal by the fusion around 

 the end of the body of the dorsal fins, the caudal and the anal. 

 Eusthenerpeton, from the Devonian of Canada, is very similar to 

 Holopty chius. 



Diplurus, from Triassic of New Jersey, and Undiua, from the 

 Jurassic of England, present the last stage in the development 

 Crossopterygians. They show an amount of specialization that 

 indicates the extent to which the group had developed and the 

 necessity for adaptation by its members to the most peculiar 

 conditions to maintain an existence. Undina was short and very 

 broad. The tail is especially broad and presented a very 

 peculiar appearance, as the end of the spinal column extended 

 beyond the broad psuedo-caudal fin formed by the posterior 

 dorsal above and the anal below. The centra of the vertebrae are 

 still unossified and the bases of the fins are reduced to single 

 pieces of bone. In both this form and Diplurus the air bladder 

 was ossified to a considerable extent so that it is preserved in the 

 cavity of the body. Diplurus was greatly shortened in the body 



