110 



COMPARATIVE ANATOMY 



become segmented off from the basal cartilage (basi- or metaptery- 

 gium) of the free fin. In some cases even this segmentation does 

 not take place, and thus the pelvis remains undifferentiated. This 

 simple condition is also met with in the ancient forms Pleura- 

 canthus and Xenacanthus, and is essentially retained in Lepi- 

 dosteus, Amia, and the Teleostei (Fig. 87). 



In Polypterus, which most nearly resembles the Devonian 

 Crossopterygii, 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 



FIG. 87. DIAGRAMS ILLUSTRATING THE PHYLOGENY OF THE PELVIS. 



A, Pleuracanthus the pelvis is here undifferentiated ft ; B, Scaphirhynchii* 

 cataphractus ; C, Polypterus bichir ; D, Necturus (Menobranchus). Bas 1 . 

 basipterygium ; Ap, its cartilaginous apophysis ; P, pelvis ; Bad, radii ; 

 Fo, obturator foramen. 



united together in the mid-ventral line (Fig. 87, C) : but even here 

 the basipterygium may remain in continuity with the pelvic plate 

 on one or both sides. In spite, however, of the rudimentary 

 character of the pelvis of Polypterus, the essential form of that of 

 the Dipnoi and Amphibia is already sketched out. 



The pelvis of the Elasmobranchii and Holocephali indicates that 

 they early branched off from the ancestral stock. Instead of 

 a small and narrow pelvic plate more or less elongated antero- 

 posteriorly, the pelvis forms a transverse bar of considerable 

 extent, developed in connection with the basipterygium (Fig. 82) 



