SQUALIN-ffi. SELACHUS. 141 



it. The habits of the fish, which Rafinesque particu- 

 larly mentions, show that it is a nocturnal feeder ; and 

 it therefore becomes highly probable that it possesses a 

 membrana nictitans, similar to all the groups we have 

 hitherto noticed; whereas both Spinax and Centring 

 according to Muller and Henle, have not that appendage 

 to the eye. At all events, we must not believe an author 

 is invariably wrong, merely because he may have com- 

 mitted occasional errors; for if we proceed on such a 

 principle, who is exempt ? We shall, therefore, retain 

 the genus Dalatias, untilit is proved false; restricting it 

 alone to the D. noctumus, and viewing it, for the pre- 

 sent, as the representation of the spiny-finned group of 

 Centrince, in the family of spiraculated sharks, to which, 

 in every thing but the absence of spiracles, it seems to 

 agree. 



(123.) The genus Selachus Cuv. is the third of the 

 SqualincB. It has several strongly-marked characters, 

 and appears altogether a very natural one. Unlike all 

 the preceding, the teeth of these sharks are conic, sim- 

 ple, and generally small ; that is, not serrated or lobed. 

 The tail, which in all the preceding genera has been 

 unequally lobed, now assumes the more regular appear- 

 ance of ordinary fishes ; its form is lunate, the two lobes 

 being nearly equal. The third character is to be found 

 in the extraordinary size of the branchial apertures, 

 which are so large as nearly to extend half way round 

 the neck. These characters are developed in the sub- 

 genera Isurus, Selachus, and Lamna f, the first of which 

 appears the true type of the group. We have now 

 arrived, however, at that extremity of the Squalince 



* Oxyrrhina Agass. evidently belongs to this group, so remarkably dis- 

 tinguished by its teeth ; but I look upon it as not sufficiently distinct from 

 Lamna to allow of sub-generic separation. Carcharodon, formed on one 

 species, is unknown to me. 



f It would appear, according to Dr. Smith, that Cuvier has overlooked 

 the spiracles of his genus Lamna, which Dr. Smith says are present in 

 that group, although extremely small. There is thus as much uncertainty 

 regarding one of M. Cuvier's genera, as in the Dalatias noctumus of Rafi- 

 nesque. May not Dr. Smith have mistaken some of the numerous pores, 

 placed on the head of certain Lamnce, for true spiracles ? We have no 

 means, at this moment, of settling this disputed point. 



