EVOLUTION OF THE VERTEBRATA. 331 



There are several evident instances of sporadic degeneracy in 

 other orders. One of these is the case of the family of the Icos- 

 teidse, fishes from deep waters off the coast of California. Al- 

 though members of the Percomorphi, the skeleton in the two gen- 

 era Icosteus and Icichthys is unossified, and is perfectly flexible. 

 Approximations to this state of things are seen in the parasitic 

 genus Cyclopterus, and in the ribbon-fishes, Trachypteridee. 



Thus nearly all the main lines of the Physoclysti are degener- 

 ate ; the exceptions are those that terminate in the Scombridse 

 (mackerel), Serranidae, and Scaridae (Pharyngognathi). 



Y. THE LIKE OF THE BATRACHIA. 



We know Batrachia first in the Coal Measures. They reach a 

 great development in the Permian epoch, and are represented by 

 large species in the Triassic period. From that time they dimin- 

 ish in numbers, and at the present day form an insignificant part 

 of the vertebrate fauna of the earth. The history of their suc- 

 cession is told by a table of classification such as I give below : 



I. Supraoccipital, intercalary and supratemporal bones present. Propodial 



bones distinct. 



Vertebral centra, including atlas, segmented, one set of segments together support- 

 ing one arch Rhachitomi. 



Vertebrae segmented, the superior and inferior segments each complete, forming two 

 centra to each arch EmbolomeH. 



Vertebral centra, including atlas, not segmented, one to each arch StegoccpJiali. 



II. Supraoccipital and supratemporal bones wanting. Frontal and propodial 



bones distinct. 



a. An OS intercalare, 



A palatine arch and separate caudal vertebrae Proteida. 



aa. No OS intercalare. 

 A maxillary arch ; palatine arch imperfect ; nasals, premaxillaries and caudal verte- 

 brae distinct Urodela* 



No maxillary or palatine arches ; nasals and premaxillary, also caudal vertebrae, dis- 

 tinct Trachystomata, 



III. Supraoccipital, intercalare and supratemporal bones wanting. Frontals 

 and parietals connate ; propodial bones and caudal vertebrae confluent. 



Premaxillaries distinct from nasals ; no palatine arch ; astragalus and calcaneum 

 elongate, forming a distinct segment of the limb Anura. 



The probable phylogeny of these orders as imperfectly indi- 

 cated by paleontology is as follows : 



* 



Probably includes the Gymnophiona. 



