FOSSIL VERTEBRATES— FISHES 4*3 



by the enormous development of the psuedo-caudal fin, formed 

 as in the case of Undina by the posterior dorsal above and the 

 anal below, with the end of the vertebral column extending out 

 beyond the two in a slender fin. The skull was relatively 

 enormous and the jaws entirely edentulous. 



Coelacanthtis, from the Carboniferous of Ohio, is another of 

 the highly-specialized Crossopterygians. The fins, scales, and 

 general contour of the body is that of a modern bony fish, and 

 it is only upon close examination that the fins were found to be 

 of the archipterygial type and the caudal fin formed in the 

 same way as the same fin in Diplurus and Undina. 



■ The Actinopterygians are separated into two groups, the Chon- 

 drosteans and the Teleocephali. The first of these groups is very 

 similar in many of its characters to the Crossopterygians, the 

 most important difference and the one that marks the separa- 

 tion of the two greater groups is the structure of the fins. 

 Instead of the lobate, or archipterygial type of fin with the 

 well-developed basals and the symmetrically-arranged radials 

 and fin rays, the basals have almost entirely disappeared and the 

 fin has developed the monoserial structure, i. e., the basal sup- 

 ports are confined to the most proximal part of the fin, and 

 the rays are developed on one side only of the supports. 1 The 

 skeleton is still cartilaginous and the scales are bony and cov- 

 ered with enamel. 



Elonicthys, from the Permian of Europe, is a typical one of 

 these forms, the body was somewhat elongate and the scales 

 were narrow. Eiirynotns from the Calciferous limestone of Scot- 

 land, Cheirodus from the Coal Measures of Scotland, and Micro- 

 don from the Jurassic of France, exhibit stages in the gradual 

 development of a great vertical expansion of the body with an 

 attendant shortening and flattening. In the last form there 

 were developed flat crushing teeth instead of the sharp, conical 

 form. 



Aspidorhynchus, from the Jurassic of Solenhofen, was an 

 elongate form much like the modern garpike in appearance 



1 See Fig. 3. Ichthyopterygium. 



