282 PISCES. 



ORDER 11. 

 CHONDROPTERYGII BRANCHIIS FIXIS, 



Or the Choudropterygii vvith fixed branchise, instead of 

 having those organs free on the external edge, and opening all 

 their intervals into a large common orifice, as is the case in all 

 the fishes of which we have hitherto spoken, have them adhe- 

 ring by this external edge in such a manner tliat they permit 

 the water to escape through as many holes pierced in the skin 

 as there are intervals between them, or, at least, that these holes 

 may terminate in a common duct, through which the water is 

 ejected. Another circumstance peculiar to these fishes is the 

 presence of little cartilaginous bows, frequently suspended in 

 the muscles opposite to the external edges of the branchisBj 

 and which may be termed branchial ribs. 



FAMILY I. 



SELACHII,— PLAGIOSTOMI, Dumer. 



This family, hitherto comprized under two genera, Squalu& 

 and Raia, has many conimon characters. The palatines 

 and post-mandibularies, alone armed with teeth, supply the 

 place of jaws, the usual bones of which are reduced to mere 

 vestiges; one single bone suspends these apparent jaws to the 

 cranium, representing at once the tympanal, jugal and tempo- 

 ral bones, and the preoperculum. The hyoid bone is attached 

 to the single pedicle just mentioned, and supports branchios- 

 tegal rays, as in ordinary fishes, although they are not so very 

 visible, externally ; it is followed by the branchial arches, as 

 usual, but neither of the three pieces compose the operculum. 

 These fishes have both pectorals and ventrals : the latter are 

 are situated behind the abdomen, and on each side of the anus. 

 Their membranous labyrinth is enclosed by the cartilaginous 

 substance of the cranium ; the sac, which constitutes part of it. 



