PELVIC ARCH 



145 



Pelvic Arch. 



Fishes. In Cartilaginous Ganoids, indications of a pelvis 

 are seen, but are very variable, even in individuals of the same 

 species. They consist of two calcified or ossified pelvic plates, 

 which correspond to portions segmented off from the. basal 

 cartilage (basipterygium) of the fin. In some cases even this 

 segmentation does not take place, and thus the pelvis remains 



FIG. 104. SIMPLE FORMS OF PELVIS AMONOST FISHES AND AMPHIBIANS. 



A, Plevracaiithus the pelvis is here not differentiated from the proximal end 

 (tr) of the basipterygium; B, Scaphirhynchus cataphraHus; C, Polypterux 

 bichir} D, Xertnrn (JftnobramJuts). Ap, apoplrysis of the basipterygium ; 

 Bas 1 , basipterygium ; Fo, obturator foramen ; P, pelvis ; Bad, radii. 



undifferentiated. This simple condition is also met with in the 

 ancient forms Pleuracanthus and Xenacanthus (Fig. 104, A, B). 



In Polypterus the pelvis shows some advance on that of 

 Sturgeons. Owing, doubtless, to the necessity of a firmer connec- 

 tion of the fin with the body-wall, the two pelvic plates become 

 united together in the mid-ventral line (Fig. 104, c). In spite, 

 however, of the rudimentary character of the pelvis of Polypterus, 

 the essential form of that of the Dipnoi and Amphibia (D) is 

 already sketched out (for Teleosts, cf. p. 159). 



The pelvis of Elasmobranchs consists of a tran verse bar extend- 

 ing between the two basipterygia, from which it has become 



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