18 COPE 



II. A maxillary arch present. 

 Dermal cranial bones and opercula present; 

 no claspers; suspensorium distinct 

 from cranium ; palatopterygoid arch 

 free ; Teleostomi. 



With our present knowledge we find the first appearance 

 of these subclasses to be contemporary ; i. e., in the earlier 

 part of the Devonic system, in the Corniferous Limestone 

 and its equivalents in other parts of the world. Spines, 

 possibly belonging to sharks (Onchus), have been found in 

 Lower Siluric (Clinton) in Pennsylvania, and the Upper 

 Siluric (Ludlow) in England, but the animals to which 

 they belong may not have been Pisces. The Devonic forms 

 referred to the Dipnoi (the Arthrodira), while probably be- 

 longing to that subclass, are not yet positively proven to be 

 such. Representatives of all four subclasses still exist. 

 The Elasmobranchii are seen in the sharks and rays; the 

 Holocephali in the Chimseras ; the Dipnoi in the lung- 

 fishes ; and the Teleostomi in the true fishes. 



The primary divisions of the fishes, as above indicated, 

 are seen in the structure of the skull. After this the 

 structure of the fins demands attention. kSome of the pe- 

 culiarities of fin structure are common to all the members 

 of a subclass, while others characterize subdivisions of the 

 same. These members consist of a dermal portion and a 

 skeletal portion. The former is external to the body-walls ; 

 the latter is internal to those walls in the case of the median 

 fins, and external in the paired fins, when present. The 

 skeletal portion consists of two sets of segments. First 

 those to which the dermal rays or folds are attached are 

 called the basilars. These are articulated to a second set, 

 the axials. The segments are then termed the baseosts and 

 axonosts. In the paired fins the latter are articulated di- 

 rectly with the elements of the shoulder and pelvic girdles. 

 Of the latter only the pubic element is present in Teleostomi. 

 In the shoulder girdle we have elements termed scapula. 



