78 Memoir Sears Foundation for Marine Research 



correspondence, more strongly so posteriorly than anteriorly; vertebral centra either with- 

 out calcification, or those in the tail region with calcareous lamellae radiating from a cen- 

 tral ring in some forms; neural spines not attached to dorsals; cranium on each side with a 

 well developed antorbital process, continuous, however, with the auditory capsule; rostral 

 cartilage single; upper jaw (palatoquadrate cartilage) attached to cranium at two points 

 {i.e., to the suborbital region and to a postorbital process), but not to the hyomandibular 

 arch which is much reduced; propterygial cartilage of pectoral without radials; heart 

 valves in 4 or 5 rows. Development ovoviviparous. 



Families. One modern family known, Hexanchidae. 



Family HEXANCHIDAE 



Characters. Either 6 or 7 gill openings; margins of all gill openings widely inter- 

 rupted at throat; eyes without nictitating folds or membranes; spiracles present; upper 

 teeth sharp, with slender, curved, primary cusps; lower teeth blade-like, quadrate or trian- 

 gular, their margins with several small cusps; caudal fin with well marked subterminal 

 notch, its axis raised but little; inner margins of pelvics either separated or briefly united 

 posterior to cloaca; no precaudal pits; dorsal fin posterior to pelvics; lower jaw with either 

 large or rudimentary labial furrows ; no upper labial furrows ; clasper of male largely en- 

 closed by a leaf-like expansion of the pelvic fin, its axial cartilage small, simple distally, and 

 attached to the basipterygial cartilage of fin by 2 small connecting segments.^ Development 

 ovoviviparous. 



Key to Genera 



la. 6 gill openings. //^;c«»c^«j Rafinesque, 1810, p. 78. 



lb. 7 gill openings. 



2a. Head narrow; snout tapering; horizontal diameter of eye considerably greater 



than distance between nostrils. Heptranchias Rafinesque, 18 10, p. 87. 



2b. Head broad; snout broadly rounded; horizontal diameter of eye considerably 



smaller than distance between nostrils. Notorynchus Ayres, 1885. 



Mediterranean, South Africa, 

 Argentina, California to Oregon, 

 Japan, China, Australia— New 

 Zealand, Indian Ocean. 



Genus Hexanchus Rafinesque, 1 8 1 o 

 Six-gilled Sharks, Cow Sharks, Mud Sharks 



Hexanchus Rafinesque, Caratt. Gen. Spec. Sicil., 18 10: 14; type species, H. griseus Rafinesque, Sicily, equiva- 

 lent to Squalus griieus Bonnaterre, 1 788. 



Generic Synonyms: 



Monofterhinus (in part) Blainville, Bull. Soc. philom. Paris, 1816: 121. 



I. For illustrations of the cartilag-es of the clasper, see Huber (Z. Wiss. Zool., 70, 1901 : pi. 27, fig. i) and Daniel 

 (Elasmobranch Fishes, 1934: 51). 



