FOSSIL FISHES. 



31 



SUB-CLASS IV. Teleostei.* 



(See Table-cases 48-oG, and Wall-cases 16-18.) 



The fishes belonging to this division have a well-ossified skeleton 

 and biconcave vertebra? ; the tail is homocercal (although in very 

 early stages of its development it has a heterocercal form, as in the 

 previous groups) ; they are usually protected externally by thin 

 imbricating, ctenoid or cycloid scales (see Fig. 53), sometimes by 

 bony plates ; in some the skin is naked. The gills are free, with 

 one external opening protected by a gill-case- 



Fig. 51. — The Freshwater Perch {Perm fluviatilis), living in the inland freshwatcrs of 



Europe and Asia. 



The Teleosteans, as at present defined, make their first appear- 

 ance in the Cretaceous period, and are divided into the following 

 groups : — 



The Acanthopterygii ; characterized by having a greater or 

 lesser number (varying according to the species) of the anterior 

 rays of the dorsal and ventral fins projecting as acutely-pointed 



* In reference to the Teleostean fishes it is necessary to remark that no very com- 

 plete or satisfactory classification of this group at present exists. It has, therefore, 

 been considered most convenient to follow, as far as possible, the only published 

 English classification of these fishes, namely, that given by Dr. GKinther. But as 

 there are a great many fossil genera and species altogether omitted by that Ichthy- 

 ologist from his catalogue, these have been grouped according to their probable 

 affinities, or as originally placed by Agassiz. A more careful revision of this great 

 Division is absolutely needful before the fossil forms can be arranged in their 

 natural relation to existing fishes. Dr. R. H. Traquair, who has devoted some 

 time to the study of this class, has, unfortunately, not yet published his views 

 thereon. The writer takes this opportunity to acknowledge his indebtedness to his 

 colleague, Dr. A. Grunther, F.R.S., from whose work, entitled "Introduction to 

 the Study of Fishes," he has made numerous extracts, especially in reference to the 

 Teleostei. 



