SELACHII. 287 



CHONDROPTERYGII TREMATOPNEONTES. 



The cartilaginous fishes with fixed gills, forming the eighth order of their 

 class in Cuvier's arrangement, are the trematopnes of Dumeril, or the ptacoidians 

 of M. Agassiz. Their gills adhere hy their outward edges in such a way that the 

 water either escapes by as many holes in the skin as there are intervals betwixt 

 them, or by a common conduit in which all these holes end. Another peculiarity 

 in the structure of these fishes consists in the frequent suspension of small cartila- 

 ginous bows in the flesh opposite to the gills, and which may be called branchial 

 ribs. The order is divided into two families. 



SELACHII. 



The selaciens of Cuvier, or plagiostomes of Dumeril, comprise the sharks and 

 rays, which have many common characters. They have the ordinary jaw bones 

 merely in a rudimentary state, their place being supplied by the palatine and post- 

 mandibular bones, which are alone armed with teeth, and are suspended to the 

 cranium by a single bone, that represents at once the petrous, jugal, and temporal 

 bones and the preoperculum. The gill-rays are attached to the os hyoides, as in 

 osseous fishes, although they are not so perceptible externally, but there is no ves- 

 tige of any of the three opercular pieces. The ventrals are situated on the hinder 

 part of the abdomen on each side of the vent. The pancreas is in form of a con- 

 glomerated gland, and not divided into distinct caeca, and the short gut is provided 

 in its lower part with a spiral valve. These fish are either oviparous or viviparous, 

 and possess well-organised oviducts. The males have curious appendages on the 

 inside of their ventral fins, whose use is not known. The following species have 

 been mentioned as inhabitants of the seas of the United States. 



Scyllii. — Set/Ilium JEclwardsii, Cuv. (Squalus cants, Mitchill; S. canicula 

 et catulus, Smith). Carchari^e. — Squalus carcharias, Penn., Mitchill, 

 Smith; Carcharias vulpes, Smith (Squalus vulpes, Mitchill) ; Squalus glaucus, 

 Mitchill, Smith ; Squalus punctatus, Idem ; Squalus obscurus, Le Sueur ; 

 Squalus littoralis, Idem. Selaches. — Selache maximus, Smith (Squalus 



