ELASMOBRANCHII. 23 



times rudimentary opercle. Skeleton cartilaginous. Skull with- 

 out sutures, mandibular suspensorium present. No air-vessel. 

 Intestine with a spiral valve. Arterial bulb with three series of 

 valves. Optic nerves united by a chiasma. Cerebral hemispheres 

 united. Gills not free, attached to skin by outer margin. Ova 

 few and large, impregnated and sometimes developed internally. 

 Embryo with deciduous external gills. Tail hetorocercal. Ven- 

 tral fins abdominal. Males with large intromittent organs or 

 claspers attached to ventral fins. Skin naked or covered with 

 minute rough scales, sometimes with spines. 



An almost perfect gradation exists from the true sharks to 

 the skates, though the notidanid sharks are somewhat removed 

 from the former. The orders are the Ichthyotomi, Notidani, 

 Asterospondyli, Cyclospondyli, Rhince and Batoidei. The first 

 of these is entirely extinct, though no fossils have been found in 

 Xew Jersey referable to this group. 1 All the others are repre- 

 sented by living forms, and possibly at least one of the Notidpni 

 may occur off our shores in deep water. 



Order NOTIDANI. 



THE NOTIDANOID SHARKS. 



Vertebral column imperfectly segmented, each segment equiv- 

 alent to 2 vertebrae and bearing 2 neural arches. Gill-openings 

 6 or 7. Dorsal fin 1. Anal present. 



This order contains the most primitive of existing sharks. 

 Families 2, recent and extinct. 



Family HEXANCHID.E. 



THE GRISETS. 



Eyes anterior or submedian. No nictitating membrane. 

 Teeth above, 1 or 2 pairs, awl-like, next 6 broader, and each 

 with several cusps, 1 enlarged. Teeth below, 6 large comb-like 



1 1 may note that a tooth of Petalodus, reported by Leidy, in Proc. Acad. 

 Nat. Sci. Phila., 1876, p. 9. is doubtfully ascribed to the New Jersey 

 Cretaceous and is therefore not likely admissible to that fauna. 



