ELASMOBRANCHIL 



547 



The embryos have external gill -filaments. They retain 



more embryonic features, e.g. the naso-buccal 

 auditory opening, than other fishes. 



groove 



and 



Order i. Pi.AGiosTOMi or SELACHII. 



With transverse ventral mouth, pre-oral rostrum, uniserial paired 

 fins, claspers, heterocercal tail, usually five pairs of open gill-clefts. 



Subdivisions. -- The 



shark and the skate are types 1>l ' '' 



of two distinct suborders : ( I ) 

 the older Selachoidei, with ap- 

 proximately cylindrical bodies 

 and lateral gill-openings, as 

 in shark and dog - fish ; 

 (2) the more modified Bato- 

 idei, with flattened bodies, 

 ventral gill - openings, and 

 pectoral fins joined to the 

 head, as in skates or rays. 



M-n steins, Carcharia.^\ 

 Squalus, Torpedo, Acanthias, 

 and others, are viviparous ; 

 Raja, Scyllinin, Cestracion, 

 and others, are oviparous. 

 In most species of Mnstelns 

 there is a placenta-like con- 

 nection between the yolk-sac 

 of the embryo and the uterus 

 of the mother. Zygicua has 

 a peculiar hammer-like head 

 expansion ; Pristi's has the 

 snout prolonged in a tooth - 

 bearing saw ; Torpedo has a 

 powerful electric organ. The 

 Greenland Shark (Lcrniargits 

 borealis] is unique in having 

 small eggs, without egg- 

 cases, perhaps fertilised in 

 the water. In the eel - like 

 deep - water Japanese Shark 

 ( Chlamydoselachns] the mouth 

 is anterior, the nostrils lat- 

 eral, 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 Notodanidae, e.g. Hexanchus 

 (six gills) and Heptanchus (seven gills), the mouth is almost inferior, the 

 vertebral column is imperfectly segmented with persistent notochord. 



FIG. 264. Young skate. From Beard. 



The yolk-sac has been cut off, the yolk-stalk i* 

 left. tn., Mouth; ol.o., nostril; e.g., exter- 

 nal gills ; a., cloaca ; c., claspers. 



