ELASMOBRA-NCHII. 3 1 



though Leidy expressly states that the type of this species, a 

 tooth only }i of an inch long, was presented to the Academy of 

 Natural Sciences of Philadelphia by Dr. Knieskern, I have not 

 located it among the collections. Leidy points out that it differs 

 from Ginglymo stoma serra in having the main point of the crown 

 inclined to one side. 



Formation and locality. Known only from the Eocene of New 

 Jersey. 



Family LAMNIDJE. 



THE MACKEREL SHARKS. 



Body stout. Mouth wide. Teeth large, sharp. Gill-openings 

 wide, all before pectorals, entirely lateral, not extending under 

 throat. Spiracles minute or absent. First dorsal large. Second 

 dorsal and anal very small. Tail slender. Caudal lunate, both 

 lobes not very unequal and upper strongly bent upward. Caudal 

 peduncle with strong lateral keel on each side. Pit at caudal 

 root. Pectorals large. Ventrals moderate. 



Large, fierce sharks in all seas, referred to eight or nine genera, 

 of which three still persist to the present time. The muscular 

 system and dentition reaches its greatest degree of specialization 

 known among sharks in this family. 



Genus ISURUS Rafinesque. 



Isurns Rafinesque, Car. Nuov. Gen. Sicil., 1810, p. 11. Type Isurus oxyrin- 



chus Rafinesque, monotypic. 

 Oxyrhina Agassiz, Poiss. Foss., Ill, 1836, pp. 87, 276. Type Lamna 



oxyrhina Valenciennes, monotypic. 

 Oxyrrhina, auct. 

 Isuropsis Gill, Ann. Lye. N. Hist. N. Y., VII, 1862, pp. 398, 408. Type 



Oxyrhina glauca Muller and Henle, designated. 

 Anotodus LeHon, Prel. Mem. Poiss. Tert. Belg., 1871, p. 8. Type Anat- 



odus agassizii LeHon, monotypic. 



Body mackerel or tunny-like, caudal peduncle slender. Snout 

 rather long, pointed. Teeth long, lanceolate, cutting-edges sharp 

 and entire, and no basal cusps. First dorsal large, entirely be- 

 hind pectoral, or nearly midway between latter and ventral. 

 Second dorsal and anal very small. Pectoral large. 



