5 o6 



FISHES. 



Fertilisation is internal. The ova are few and large. Large 

 egg purses are common, but some Elasmobranchs are 

 viviparous. The embryos have external gills. 



Subdivisions. The shark and the skate are types of two distinct 

 suborders : ( I ) the older 

 Selachoidei, with approxi- 

 mately cylindrical bodies and 

 lateral gill openings, as in 

 shark and dog fish ; (2) the 

 more modified Batoidei, with 

 flattened bodies and ventral 

 gill openings, as in skates or 

 rays. 



Special Forms. Mustelus, 

 Carcharias, Squalus, Torpedo, 

 Acanthias, and others, are 

 viviparous ; Raja, Scy Ilium, 

 Cestracion, and others, are 

 oviparous. In two species 

 of the genera first named, 

 there is a placenta-like connec- 

 tion between the yolk sac of 

 the embryo and the uterus of 

 the mother. Zygcena has a 

 peculiar hammer-like head 

 expansion ; Pristis has the 

 snout prolonged into a tooth- 

 bearing saw ; Torpedo has 

 a powerful electric organ. 



History. The Elasmo- 

 branchs appear in the Upper 

 Silurian, are very abundant 

 from the Carboniferous on- 

 wards, but are now greatly 

 out-numbered by the Bony 

 Fishes. An increasing calci- 

 fication of the axial skeleton 

 is traceable through the ages, 

 and in some of the ancient 

 forms the exoskeleton was 

 greatly developed, often in- 

 cluding long spines or ichthyo- 

 dorulites firmly fixed on the isj^^^j^?^ ^^'/ wtSSal 

 dorsal fins or on the neck, gflfe ;*., cloaca ; *^' daspcrs. . 

 Among the most remarkable 



extinct genera is Pleuracanthiis, from Carboniferous to lower Permian. 

 It had a terminal mouth, a naked body, a continuous dorsal fin, a 

 symmetrical tail, and pectoral fins with an arrangement of rays 

 resembling that in the biserial " archi-pterygium." 



FIG. 170. Young Skate. 

 (From BEARD.) 



