148 CLASSIFICATION OP FISHES. 



Squatina, Crossorhinus, Sac, by the mouth being ter- 

 minal, and the eyes vertical, but still further resembles 

 the first, in the lobes of the tail being nearly equal; so 

 that there can be but little doubt of its being the chi- 

 ronectiform type of the circle of spine-finned sharks. 

 The front view of the head, as delineated in M. Les- 

 son's AtlaSy gives the lateral ridges or elongated lobes au 

 appearance of horns ; but this merely results from the 

 peculiar position in which the head is drawn. 



(128.) The genus Mustelus is the fourth of this sub- 

 family ; and although, in its general form, it has a close 

 resemblance to Galeus, it is yet distinguished from all 

 the other spiraculated sharks by having the teeth flat and 

 tessellated, like the rays and the genus Pristis : it is on 

 his accountt that we consider Mustelus as the represent- 

 ative of a genus connected, in the most perfect manner^ 

 with that of Centrina, through the medium of Cestracion^ 

 which unites in itself the chief characters of both, 

 joined to a peculiarity of its own. 



(129.) Regarding the fifth primary group of the 

 CentrincB, much uncertainty prevails, on account of tlie 

 conflicting opinions of Rafinesque and Cuvier as to the 

 question whether Heptranchias has no spiracles, or 

 whether they really do exist, as asserted by the latter. 

 It is clear, however, that even if Lacepede, rather than 

 Rafinesque, is in error on this point, and that Heptran- 

 chias is but a sub. genus of Hexanchias, the latter name 

 has the priority over Cuvier's Notidanus, — having been 

 published seven years before.* Leaving, therefore, the 

 presence or absence of spiracles in Heptranchias to be 

 determined hereafter, we may state that the genus 

 Hexanchus is distinguished by having no second dorsal 

 fin, and that it seems to contain two sub-genera : Hex- 

 anchus proper, having a depressed and rounded muzzle, 

 and six wide branchial apertures ; and Heptranchias, 



* If there is an error in attributing no spiracles to the sub-genus Hep- 

 tranchias, that error belongs to Lacepede, and not to Rafinesque, who 

 founds his generic characters entirely upon Lacepede's account of his 

 Squaie perion {Hist, des Pais&ons, p. 2200, without having seen the specie* 

 himself, which he does not describe. 



