So6 



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. 



Siibdivisio}is. — The shark and the skate 

 suborders: — (l) 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. — Mustehis^ 

 Carcharias^ Squalus. Torpedo^ 

 Acanthias, and others, are 

 viviparous ; Raja^ Scyllittm, 

 Cesiracioit, and others, are 

 oviparous. In two species 

 of the genera first named, 

 there isaplacenta-like connec- 

 tion between the yolk sac of 

 the embryo and the uterus of 

 the mother. Zygana 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- 

 doruhtes firmly fixed on the 

 dorsal fins or on the neck. 

 Among the most remarkable 



are types of two distinct 

 m. ol.o. 



Fig 



1 70. — Young Skate 

 (From Beard.) 



(The yolk ^ac tias been cut off, the yolk stalk 

 is left.) ;//., Mouth ; ol.o.^ nostril ; e.g.., external 

 gills; a., cloaca; c, cLaspers. 



extinct genera is Pkiiracanthus. 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." 



