54 REPORT OF NEW JERSEY STATE MUSEUM. 



Family LAMNID^. 



The Mackerel Sharks. 



Body stout. Mouth wide. Gill-openings wide, all in front of 

 pectorals, and entirely lateral, or not extending under the throat. 

 Spiracle minute or absent. First dorsal large. Second dorsal 

 and anal small. Tail slender and pit at root of caudal. Pectorals 

 large. Ventrals moderate. 



Numerous fossil species are known. In this family, the 

 muscular system and dentition reaches its highest degree of 

 specialization known among sharks. 



Key to the genera. 



a. Teeth without basal ciisps, long, flexuous, acute. isurus 



aa. Teeth each with i or 2 basal cusps. lamna 



Genus Isurus Rafinesque. 



The Porbeagles. 

 Isurus dekayi (Gill). 



Plate i. 



Mackerel Porbeagle. 



Body cylindrical, fusiform. A carina on each side of caudal 

 peduncle, highest in middle. A deep indentation in upper and 

 lower surfaces of caudal peduncle in shape of horseshoe. Head 

 small, with blunt pyramidal snout. Eyes moderate, lateral. 

 Teeth various, in 3 to 5 rows. Anterior mandibular teeth 

 largest, irregularly lanceolate, external and sometimes internal 

 edge rounded and waved. Margins with finely elevated crest, 

 smooth, absolutely serrate under lens, flattened in front, and 

 rounded, approaching a triangular form behind. Occasionally 

 recurved, tips turning slightly outwards. Front upper teeth long. 



