THE ORDERS OF FISHES. 427 



cloaca) of the gut, the genital and urinary ducts also open. 

 The ventricle of the heart has an anterior auxiliary region 

 — a contractile conus arteriosus. The males are provided 

 with copulatory modifications of the hind limb, known 

 as claspers. Fertilisation is internal. The ova are few 

 and large. Large egg-purses are common, but some Elas- 

 mobranchs are viviparous. The embryos have external 

 gills. 



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

 sub-orders: — (i) the older Selachoidei, with approximately 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, Scyllium, Cestracion, and others, are 

 oviparous. In two species of the genera first named, there is a placenta- 

 like connection between the yolk-sac of the embryo and the uterus of 

 the mother. Zygcena has a peculiar hammer-like head expansion ; 

 Fristis has the snout prolonged into a tooth-bearing saw ; Torpedo has 

 a powerful electric organ. 



History. — The Elasmobranchs appear in the Upper Silurian, are very 

 abundant from the Carboniferous onwards, but are now greatly out- 

 numbered by the Bony Fishes. An increasing calcification of the axial 

 skeleton is traceable through the ages, and in some of the ancient forms 

 the exoskeleton was greatly developed, often including long spines or 

 ichthyodorulites firmly fixed on the dorsal fins or on the neck. Among 

 the most remarkable extinct genera is Pleuracanthus, from Carboniferous 

 to lower Permian. It had a terminal mouth, a naked body, a con- 

 tinuous dorsal fin, a symmetrical tail, and pectoral fins with a serial 

 arrangement of rays somewhat like that in Ceratodus. A Japanese 

 shark Chlamydoselachtis, allied to forms in the Devonian, is said to be 

 the oldest living fish. 



The Holocephali are represented by the sea-cat or Chimara from 

 northern seas, and Callorhynchus from the south. , There is a fold or 

 operculum covering the gill-clefts and leaving only one external opening 

 on each side ; the jaws are rigidly fixed to the cartilaginous skull ; the 

 skin is naked ; the anus, the Mtillerian and urinary ducts open sepa- 

 rately. Otherwise the Holocephali resemble Elasmobranchs, and may 

 be regarded as a sub-order. 



Order II. Dipnoi. 



The mud-fishes or double-breathers, including Ceratodus 

 and Protopterus. 



General Characters. — The body is covered with com- 

 paratively soft scales ; the skeleton is in part cartilaginous ; 



