NATURAL SCIENCES OP PHILADELPHIA. 489 



e9t period down to our own days. These vestiges are almost solely the more or 

 less complete remains of teeth and spines. It is therefore by ho means demon- 

 strated that all such remains are indications of the pertinence of the speciesof 

 which they are the witnesses, to the present family. All these remains require 

 to be re-examined with reference to the present views held by naturalists re- 

 garding the nature of families. Such an examination will doubtless result in 

 the disseyerment of some of the genera known from such remaius, from the 

 family of Heterodontoids. 



That family of Heterodontoids as now restricted, is distinguished among all 

 the others representatives of the order by the peculiar form of the body and 

 head. While in all the other recent sharks, the head is depressed and the 

 snout, above nearly parallel or on the same plane with the upper surface of the 

 head, in the Heterodontoids, the head is elevated, the sides vertically expanded 

 and the snout deflected downwards. The teeth form another very characteristic 

 feature, those towards the front being incisorial or digitated, while those on the 

 sides are molar and arranged in oblique whorls. Each dorsal is in front provi- 

 ded with a spine mostly enveloped in its substance, but with its point exposed. 

 The simple teleological adaptation of the teeth of the ancient representatives 

 of the Squali and their concurrence with spines have been the cause of the 

 reference of those remains to the Cestracionts or Heterodontoids. 



There are now known four living species of the family of Heterodontoids 

 which appear to belong to three distinct genera, chiefly separated on account of 

 the modifications of dentition, and the size of the branchial apertures. The 

 several may be briefly distinguished by the following characters : 



I. Branchial region higher than long, the slits being elongated.. Heterodontus. 



II. Branchial region longer than high, slits little elongated. 



a. Molar teeth rounded and carinated along the middle. Dor- 

 sals little produced towards the anterior angle Tropidodus.* 



/J. Molar teeth flat and closely contiguous. Dorsals produced 



backwards to the anterior angle Gyropleurodus. 



Genus GYROPLEURODUS Gill. 



Cestracion sp. Girard. 

 Heterodontus sp. Gill. 



Gyropleurodus Gill, Proc. Academy of Natural Sciences of Phila., vol. xiv. p. 

 330, July (Sep.), 1862. 



Body triquetrous in front, behind the anus attenuated and compressed towards 

 the caudal fin. 



Head short and high, broad, but with subvertical sides, with the forehead 

 very declivous from eyes, and with the snout wide and transverse, but promi- 

 nent. Two blunt diverging ridges are continued from each side of the snout 

 and abruptly merge into the more conspicuous superciliary ridges, the interval 

 between which is nearly plane. Inferior surface of head plane. 



Eyes entirely lateral, protected above by the superciliary ridge. 



Mouth inferior, but near the front, with the cleft semi-elliptical but externally 

 transverse and simply arched in front. The branches of the jaws are separated 

 by au ovate-triangular space, wide and rounded in front and thence curved out- 

 wards to the angles. 



Teeth in front digitated with three or five cusps, quincuncially distributed in 

 rows slightly converging towards the middle; in the upper jaw on the sides, 

 molars oblong aud flattened, arranged in about four oblique whorls, uniform 

 or increasing backwards, except the last, which is smallest. Ou the sides of 



* With this genus I am only acquainted through the figure and description of Valenciennes, 

 who describes its type as the Cestracion pantherinus in the Ichthyology of the Venus, Voyage 

 autour du monde sur le fregate la Venus, Zoologie, p. 350. Ichthyologie, pi. x. fig. 2. 



1862.] 



