THE SKELETON IN FISHES. PAIRED FINS. 127 



PELVIC GIRDLE. 



In Elasmobranchs the pelvic girdle consists of a short 

 ventral rod of cartilage representing the ischium and pubis, 

 which does not send up dorsal iliac processes. In Ohimaera 

 the pelvic girdle has a flattened pointed iliac portion, and 

 ventrally an unpaired movable cartilaginous plate which 

 bears hooks and is supposed to be copulatory in function. 

 Claspers of the usual type are present as well. The Dipnoi 

 have a primitive kind of pelvis in the form of a cartilaginous 

 plate lying in the mid ventral line and drawn out into three 

 horns anteriorly. In Ganoids the pelvis has almost entirely 

 disappeared, though small cartilaginous vestiges of it remain 

 in Polypterus. In Teleosteans even these vestiges are gone, 

 and in these fish and Ganoids the place of the pelvis is taken 

 by the enlarged basi-pterygia (meta-pterygia) of the fins. 



PAIRED FINS 1 . 



As regards the origin of the limbs or paired fins of fishes 

 there are two principal views. One view, that of Gegenbaur, 

 considers that limbs and their girdles are derived from visceral 

 arches which have migrated backwards. The other view, which 

 probably now has the greater number of supporters, considers 

 that the paired fins of fishes are of essentially the same nature 

 as the median fins. 



According to Gegenbaur's view 2 the archipterygium of 

 Ceratodus (fig. 20) represents the lowest type of fin ; it con- 

 sists of a central cartilaginous axis bearing a large number of 

 radialia. The dorsal or pre- axial radialia are more numerous 

 than the ventral or postaxial, and at the margin of the fin 3 the 



1 A. Smith Woodward, Nat. Sci. vol. 1. 1892, p. 28. Further references 

 are here given on the literature of the subject. 



2 C. Gegenbaur, Ueber das Archipterygium, Jena Zeitschr. der Wirbel- 

 thiere, 2 e Heft, 1873, vol. 7, and MorphoL Jahrb. xxn. 1894, p. 119. 



3 The fins of Ceratodus are very variable, no two being exactly alike. 

 Sometimes even the main axis bifurcates. See W. A. Haswell, Linn. 

 Soc. N. S. Wales, vol. vn. 1882. 



