24 PLAGIOSTOMES. 



elements of the temporal bone. In its simplicity, in the plagios- 

 tomes, it participates with the character of the rest of their cartilagi- 

 nous cranium, in which the several distinct bony elements found 

 in the reptiles and osseous fishes cease to be recognisable. Of the 

 several bones concerned in the formation of the jaws, we have to 

 seek, in the simple cartilaginous arches of the skate, the pterygoid, 

 palatine, maxillary, intermaxillary, mandibular, and premandibular 

 bones ; and to these may be likewise added the labial cartilages, 

 which are so largely developed in the cyclostomous fishes. Are all 

 these elements combined in the dentigerous cartilages of the skate ? 

 or if not all, which ? These questions have been differently answered 

 by different comparative anatomists. The essential character of the 

 pterygoid and palatine bones manifests itself, in the oviparous classes, 

 in the formation of buttresses extending between the vomer and the 

 articular pedicle of the jaw. In the Carcharias glaucus, or blue-shark, 

 and in the Lamna, or porbeagle, a distinct flattened process of car- 

 tilage extends from each side of the vomerine region of the skull, 

 and abuts against the proximal extremity of the pedicle, and the 

 contiguous part of the cranium. These processes, I regard as 

 analogous to the palato-pterygoidean buttresses. In the common 

 torpedo, there is a distinct cartilage in the corresponding situation, 

 and in the Brazilian torpedo, (1) Dr. Henle has discovered a second 

 broader cartilage, anterior and internal to the pterygoidean pedicle, 

 and which he considers to be the analogue of the true palatine 

 bone ; this cartilage has not been found separately developed in any 

 other plagiostome. 



With respect to the labial cartilages these are wanting, accord- 

 ing to Miiller, in the following subgenera of the ray tribe, Raia, 

 Trygon, Rhinohates, Cephaloptera, and Myliohates ; they are also 

 absent in the Carcharias, Cestracion, and Pristis among the sharks. 

 They are present in the Tope {Galeus), which has one on each side 

 of the upper lip; in Scymnus there are corresponding cartilages, which 

 are elongated and extend below the angle of the mouth ; in Scyllium 

 and Mustelus, there is one on each side of both upper and lower lips ; 



(1) Torjicclo huziUeniis, the type of the sub-genus Narcine of Miiller and Henle'. 



