W On the characters of Cartilaginous Fishes. 



root of this fin. It seems that there exist Sharks, very nearly 

 related to Carcharias, but wanting this dimple, a character 

 which would of itself constitute a genus. 



b. Teeth pointed, with lateral denticles, like the teeth of Scyllium. 



1. Triaenodon, Nob. On each side of the teeth there is a denticle, which 



on the exterior side of most of those in the lower jaw is double. — 

 Caudal fin as in Carcharias, with a dimple at the root. (1 sp.) 



2. Leptocharias, Andr. Smith. Teeth numerous, one or two lateral denti- 



cleson each side. The dimple near the caudal fin wanting, and the 

 inferiorlobe of the fin scarcely indicated. The nasal valve elongated 

 into a cirrhus. (1 sp.) 

 We are as yet unacquainted with the valves of the intes- 

 tine in these two genera. 

 B. Possessing spiracles. 



a. Teeth flat, sharp, serrated or not serrated. 



1 . Galeocerdo, Nob. Teeth strongly serrated on the exterior edge, finely 



on the interior. Spiracles small. A dimple on the root of the tail : 

 the upper lobe of the caudal fin elongated, with two incisions. — 

 Valve of the intestine as in Carcharias. (2 sp.) 



2. Loxodon, Nob. Teeth without serrature, as in Scoliodon Spiracles 



very small. Valve of intestine, dimple of the tail, and caudal fin as 

 in the preceding genus, but the upper lobe of that fin with only one 

 incision. (1 sp.) 



3. Galeus. Teeth, in both jaws, serrated on the exterior edge, inclined 



outwardly. Tail as in Carcharias, but wanting the dimple. Valve 

 of intestine in a spiral. (1 sp.) 



b. Teeth pointed, as in Scyllium. 



Triakis, Nob. Teeth as in Triaenodon. Dimple of the tail wanting ; 

 the inferior lobe of the caudal fin not distinct. (1 sp.) 



c. Teeth pavement-like, or presenting a general continuity of surface, as in 



the Skates. 

 . Mustelus. Spiracles large. Inferior lobe of the caudal fin very short. 

 Membrana nictitans only rudimental. Valve of intestine in a spi- 

 ral. (1 sp. 3 var.) 



In none of the following genera is there any trace of a mem- 

 brana nictitans. 



The Lamnoidea form the second group, of which the fol- 

 lowing characters may be given. 



Fam. LAMNOIDEA. 



Branchial openings large, all situated before the pectoral fins. Spiracles 

 small. Caudal fin in the form of a crescent, with a lateral keel and a dis- 

 tinct dimple, and the small anal and second dorsal fins opposite each other. 



1. Lamna. Head pointed, conic; spiracles extremely small, disco- 



vered by Dr. Smith ; teeth long, pointed, with two lateral denticles, 

 indistinct or wanting in young individuals. The third, sometimes 

 also the fourth or fifth tooth of the upper jaw is smaller than the 

 rest. (2 sp.) 



2. Oxyrrhina, Agass. Teeth long and thick, like nails, without lateral 



denticles, the anterior ones introverted; the third tooth of the up- 

 per jaw is small and short. (2 sp.) 



*This Shark is the Lamia of Rondelet, and seems to be the Carcharias 

 verus of Agassiz. 



