no Memoir Sears Foundation for Marine Research 



pectoral ; gill arches without rakers and not interconnected by a sieve of modified denticles} 

 nostrils entirely separate from mouth, without barbels 5 spiracles present or absent} lower 

 eyelid without nictitating fold or membrane} both jaws with labial furrows at corners} 

 teeth large, few in number, awl- or blade-like, with i cusp} head of normal shape (not 

 widely expanded)} rostral cartilages 3, united at tip} metapterygium of pectoral with 

 about 3 times as many radials as mesopterygium, but the latter nearly as large as former} 

 meta- and mesopterygia not separated by a foramen } heart valves In 3 rows. Development 

 ovoviviparous. 



Genera. One of the members of this family, set apart from all the others by its trian- 

 gular, serrate teeth, has long been considered as representing a well marked genus, Car- 

 charodon. The remaining isurids fall in two groups: (A) Very stout-bodied} first dorsal 

 originating over or anterior to inner corner of pectoral when latter is laid back} first two 

 teeth in each jaw similar in shape to subsequent teeth} caudal fin (so far as known) with a 

 secondary longitudinal keel on either side below the primary keel formed by the lateral 

 expansion of the caudal peduncle. (B) More slender-bodied} first dorsal originating defi- 

 nitely posterior to inner corner of pectoral} first two teeth in each jaw noticeably more 

 slender and more flexuous than the others} without secondary keels. It seems reasonable 

 to accept the difference between the two groups, and especially the presence or absence 

 of the secondary caudal keels, as sufficiently important for generic separation. This course 

 is followed here. Fortunately there has been no need to coin a new generic name in either 

 case. 



Key to Genera 



I a. Upper teeth broadly triangular with serrate edges. 



Care Aar o<io« Agassiz, L., 1838, p. 133. 



lb. Upper teeth slender, with smooth-edged cusps. 



2a. First 2 teeth in each jaw similar in shape to the succeeding teeth} most or all of 

 teeth with lateral denticles in most species, and perhaps in all (lateral denticles in 

 young specimens may be so small as to be difficult to recognize} they may even be 

 lacking on some of the teeth) } origin of ist dorsal about over or anterior to inner 

 corner of pectoral when latter is laid back} anterior part of caudal fin with a sec- 

 ondary caudal keel on either side below the primary keel formed by the lateral 

 expansion of the caudal peduncle. Lamna Cuvier, 1 8 1 7, p. 1 1 1 . 



2b. First 2 teeth in each jaw noticeably more slender and more flexuous than the 

 others} no lateral denticles on any of the teeth} origin of ist dorsal definitely 

 posterior to inner corner of pectoral when latter is laid back} caudal fin without 

 secondary keels, with only the primary keels formed by the lateral expansion of 

 the caudal peduncle. Isurus Rafinesque, 1 8 lO, p. 1 23. 



