ELASMOBRANCHII, 567 
Large egg-purses are common, but some Elasmobranchs 
are viviparous. ‘The embryos have gill-filaments projecting 
out of the gill-clefts, so-called external gills. They are 
really elongated internal gills. Elasmobranchs retain more 
embryonic features, e.g. the naso-buccal groove and auditory 
opening, than other fishes. 
Order 1. PLAGIOSTOMI or SELACHII 
With transverse ventral mouth, pre-oral rostrum, uniserial paired 
fins, claspers, heterocercal tail, usually five pairs of open gill-clefts. 
Subdivisions.—(1) The older Selachoidei, with approximately 
cylindrical bodies and lateral gill-openings, as in shark and dogfish ; 
(2) the more modified Batoidei, with flattened bodies, ventral gill- 
openings, and pectoral fins joined to the head, as in skates or rays. 
Mustelus, Carcharias, Squalus, Torpedo, Acanthias, and others, are 
Fic. 302.—Lateral view of dogfish (Scy/zum catulus). 
Note ventral mouth with naso-buccal groove, heterocercal tail, and 
unpaired fins. gs., Gill-slits; Zc., pectoral fins; 4v., pelvic 
fins. 
viviparous ; Raja, Scylléum, Cestracton, and others, are oviparous. In 
most species of AM/ustelus there is a placenta-like connection between 
the yolk-sac of the embryo and the uterus of the mother. In several 
viviparous genera long filaments are developed from the inner surface 
of the uterus which secrete a nutritive fluid. In some cases the 
nutriment seems to be afforded by degeneration of the uterine wall. 
In Acanthias vulgaris there is no nutritive material, and the young 
are unattached. ‘This is intermediate between oviparous and specialised 
placental conditions. Zygena has a peculiar hammer-like head expan- 
sion; Se/ache reaches a length of 40 ft. ; Przstés has the snout prolonged” 
in a tooth-bearing saw ; Zorgedo has a powerful electric organ. The 
Greenland Shark (ZLemargus borealis) is unique in having small eggs, 
without egg-cases, perhaps fertilised in the water. In the eel-like 
deep-water Japanese Shark (Ch/amydoselachus) the mouth is anterior, 
the nostrils lateral, the vertebral column is imperfectly segmented, _ 
there is a slight opercular fold, and there are six pairs of gill-openings 
and arches. In the large viviparous Notidanide, e.g. Hexanchus (six , 
