54. REPORT OF NEW JERSEY STATE MUSEUM. 
Family LAMNIDA. 
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 cusps, long, flexuous, acute. ISURUS 
aa. Teeth each with I or 2 basal cusps. LAMNA 
Genus Isurus Rafinesque. 
The Porbeagles. 
Isurus dekayi (Gill). 
PLATE 1. 
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, 
