CHAPTER VI. 



SUB-CLASS ELASMOBRANCHII.* 



Fishes with a cartilaginous endosJceleton, 'placoid scales, and 

 abdominal pelvic fins provided with claspers in the male. There 

 is a conus arteriosus, an optic chiasma and a spiral valve in the 

 intestine. There is no air-hladder. The eggs are large, and, except 

 in Laemargus, provided with a horny case. In the embryo the 

 gills project from the gill clefts as filaments. 



The Elasmobranchii or cartilaginous fishes include the sharks 

 and rays. With the exception of one or two sharks and a few 

 rays they are entirely marine forms. They are remarkable for 

 possessing more features which are embryonic in the higher 

 purely terrestrial Vertebrata than any other group of fishes. 

 Of such may be mentioned the oro-nasal groove, the opening 

 between the membranous labyrinth and the exterior, the un- 

 covered gill apertures, the open spiracle, the cartilaginous 

 skeleton, the opening between the pericardium and the body 

 cavity. Lastly they are the only fishes which possess eggs 

 containing so much yolk that the whole development is embryonic. 



Only two species of shark are known to be exclusively 

 inhabitants of fresh-water {Carcharias nicaraguensis and gan- 

 geticus), but several ascend large rivers, e.g., the Tigris and 

 Ganges, to a considerable distance. Most Selachians are pelagic 

 or shore forms, and some descend to great depths [Scyllium has 

 been taken at 700, Chlamydoselachus at 100-150, Centroscyllium 

 at 245, Pnstiurus at 500, and Centrophorus at 345-500 



* See Giinther, Day, Jordan and Evermann, Bridge, Boulenger, cited 

 under Pisces ; J. Miiller and J. Henle, Systematische Beschreibung der 

 Plagiostomen, Berlin, 1839. F. M. Balfour, Development of Elasmohranch 

 Fishes, London, 1878. 



