132 



sub-claSS elasmobra\( nil. 



the cloaca and carrying at its outer end the skeleton of the 

 fin. The main portion may be called the ischio-pubic portion, 

 the short process external and anterior to the limb being all 

 that can be compared to an ilium. 



The fin presents two basal cartilages of which the larger 

 is the metapterygium and carries most of the peripheral somac- 

 tids ; the propterygium is small. In the male the metaptery- 

 gium is continued to form the skeleton of the clasper. 



Here as in the fore-limb there is some variation in the number of basal 

 cartilages, mainly manifested in extinct forms. 



The brain * (Figs. 77, 78) of 

 Plagiostomes presents great varia- 

 tions but is always characterised by 

 the following features : (1) The 

 olfactory lobes, which may be 

 placed some distance in front of 

 the cerebrum and connected with 

 the latter by a long stalk, or may 

 be placed quite close to the cere- 

 brum, are connected with the cere- 

 brum laterally and somewhat 

 dorsally ; (2) the cerebrum is 

 unpaired though internally it 

 possesses two lateral ventricles, each 

 of which is usually continued into 

 its olfactory lobe ; it is not sharply 

 marked off from the thalamencepha- 

 lon ; (3) the epiphysis is elongated, 

 its front end lying over the cerebrum either beneath or 

 just behind the cranial fontanelle (in Baja in the sub- 

 cutaneous tissue above the fontanelle) ; (4) the optic 

 nerves form a chiasma, and the infundibulum has lobi 

 inferiores and an infundibular gland (saccus vasculosus) ; 

 (5) the cerebellum is large and its surface is frequently con- 

 voluted. There is said to be a paraphysis, but there is 



* N. V. Miklucho-Maclay, Beitrdge zur vergl. Neurologic d. Wirhelthiere, 

 Leipzig, 1870. J. V. Rohon, " Das Centralorgan des Nervensystems der 

 Selachier," Denkschr. d. Wiener Akad. Math., etc., 38, 1877. L. Edinger, 

 " Unters. lib. d. vergl. Anat. des Gehirns, Abh. d. Senckenbergischen naturf. 

 Gesellschaft, i. and ii. 1888 and 1892. 



FIG. 76. — Skeleton of right pectoral 

 fin of Pleuracaiithus [Xenacan- 

 thus) decheni (after A. Fritsch, 

 from Gegenbaur). 



