SKELETON OF THE PAIRED FINS 



353 



on as large as possible a number of individual specimens of each 

 species. 



The skeleton of the pelvic girdle arises in a manner similar in its 

 main features to that of the pectoral girdle. It is however character- 

 istic of Elasmobranchs (except Holocephali) that the portion of the 

 girdle dorsal to the attachment of the limb undergoes atrophy in 

 later stages of development. As in the pectoral fin an axial 

 cartilage appears with fin rays sprouting from its external side. 

 Here also a separate cartilage develops anteriorly with a few rays 

 attached to it but it is doubtful whether it is justifiable to homo- 

 logize this in detail either with the propterygium or the meso- 

 pterygium of the pectoral fin. 



The cartilaginous skeleton of the clasper arises in continuity 



Fig. 169. — Pectoral fin skeletons of: A, Ceratodus (Semon) ; B, Pleuracanthus (Fritscli) ; 

 C, (lentrophorus embryo (Braus) ; D, Acanthias (Gegenbaur) ; E, Cladosdache (Bash-' 

 ford Dean) ; F, Polypterus larva (Budgett) ; G, Polypterus (Wiedersheim). 



The outer or preaxial side of the limb is to the left, except in A. 



with the rest of the fin skeleton and appears to consist of the tip of 

 the limb axis with possibly a few modified rays. The claw-like 

 structures are simply modified placoid scales. 



Teleostomi. — As regards Polypterus, commonly regarded as the 

 most archaic of existing Teleostomes, our knowledge of the develop- 

 ment of the limb skeleton is fragmentary. In the larva of stage 36 

 (Fig. 197) the skeleton of the pectoral limb is in the form of a thin 

 lamina of cartilage with small irregularly scattered perforations. 

 This is connected with a shoulder girdle rudiment consisting of a 

 simple curved rod of cartilage. In the 30 mm. larva described by 

 Budgett (1902) the girdle has become shortened into a compact block 

 of cartilage and the cartilaginous plate lying within the limb itself 

 has become thickened along its anterior and mesial edges. These 

 thickened portions are separating off to form the rod-like " proptery- 



VOL. II 



2 A 



