490 PROCEEDINGS OF THE ACADEMY OP 



the lower jaw also molars oblong, with flattened crowns, and arranged n 

 transversely oblique whorls, but decreasing backwards. 



Upper lip narrow, emarginated in the middle, and with a median furrow ; 

 lower lip obsolete at middle, and developed laterally as a transverse flap, 

 covered at the angle of the mouth by a duplicative or flap above. 



Nostrils with a broad flap on the internal side, separated by a furrow from 

 the lip, and with a roll of skin curled inwards on the external aide. 



Branchial apertures five, small and regularly decreasing in size, the bran- 

 chial region being longer than high. 



Dorsal fins rather large, similar in form, but first rather larger than second ; 

 each with a large compressed trihedral spine enveloped in the front margin, 

 but separated partly by a slit and groove from the rest of the fin ; the latter is 

 recurved backwards towards the "anterior angle," which projects about as fur 

 behind as the posterior. 



The present genus is an interesting addition to the living representatives of 

 the ancient family of Heterodontoids, to which it belongs. It decidedly differs 

 from Heterodontus* in the development of the jaws, dentition and the size of the 

 branchial apertures. In the latter genus, the branches of the lower jaw are at 

 first contiguous and diverge from each other at an acute angle, while in front 

 of the oblique whorls of molars and between the acute teeth of the front, 

 which encroach on the sides, a cordifoim area exists. The lateral or molar 

 teeth are numerous and arranged in oblique whorls, which rapidly increase in 

 size to the fifth, behind which they again decrease. The branchial apertures 

 are also comparatively large, the first being longer than the length of the bran- 

 chial region. In Gyropleurodus, the branches of the lower jaw are widely sepa- 

 rated by an interval rounded in front and becoming wider behind, the sides 

 themselves being curved outwards; the acute teeth are confined to the front, 

 and the molar teeth are few and disposed in about four whorls, the first three 

 of which slightly decrease, while the fourth is almost rudimentary. The bran- 

 chial area is also almost oblong. There will be few, I think, who will not at 

 once admit the value of these characters and allow their generic importance. 

 Upon differences of much less value, many acknowledged genera of Squali have 

 already been established. 



The genus Tropidodua, established for the reception of the Cestracion panthe- 

 rinus of Valenciennes, differs from Gyropleurodus, at least in the keeled and 

 rounded molar teeth of the sides of the jaw, and the smaller dorsal fins, the 

 anterior angles of which project comparatively little backwards. f 



Gyropleurodus francisci Gill ex Girard. 



Cestracion francisci Girard, Proc. Academy of Natural Sciences of Phila., vol. 



vii. p. 196, 1854. 

 Cestracion francisci Girard, Explorations and Surveys for a Railroad Route, &c, 



vol. x. Fishes, p. 365. 

 Heterodontus francisci Gill, American Journal of Science and Art, ser. 2, vol. 



xxx. p. 281, 1860. 

 Gyropleurodus francisci Gill, Proc. Academy of Natural Sciences of Phila., 



vol. xiv. p. 330, 1862. 



(On account of the interest attached to the representatives of the family of 

 Heterodontoids, the following extended description of Gyropleurodus francisci is 

 submitted.) 



* The following is the diagnosis of the genus Heterodontus, published by Blainville: 



6. Heterodontus. Car. Dent, heteroclitis; Insn. nullis; P. S. [Pinnis superioribus vel dorsalibus.j 

 2 ut in prajcedeiiti ; P. A. magna; P. C. fere ut in praeced. 



Spec. Philippi. 



f "Le bouche n'est pas ties-large, elle parte en avant einq a six rangs de petites dents aigues, 

 ayant a. la base deux petitB talons epineux, puis viennent sur les cotes des machoires six rangees de 

 molaires arrondies et carenees sur le milieu." Valenciennes. 



[Oct. 



