444 FISHES CHAP. 



Scarcely anything is known of the habits of the JSTotidanidae 

 and the Ohlamydoselachidae. It is evident that they are closely- 

 related forms, and from the unusual number of their gill-clefts 

 and branchial arches, and the condition of the vertebral column, 

 it is also obvious that they are the most archaic of modern 

 Selachians. 



Fig. 252. — Chlainydoselachus anr/ioineus. (Fi'om Giintlier.) 



Fam. 3. Heterodontidae (Bullhead Sharks). — Head large and 

 high, with a blunt snout projecting but little in front of the 

 small and almost terminal mouth, and with prominent supraorbital 

 crests. Trunk thick-set and somewhat trihedral, covered with 

 fine shagreen. ISTostrils ventral but nearly terminal, with oro- 

 nasal grooves. Spiracles small, beneath the eyes. Two dorsal 

 fins, each with a spine in front, the first opposite the interval 

 between the pectorals and pelvics, the second in front of the 

 anal. Vertebral centra asterospondylic when fully developed. 

 Palato-quadrate cartilages with an extensive articulation with 

 the sides of the preorbital regions of the cranium, the normal sus- 

 pensoria of a hyostylic skull (hyomandibular cartilages) taking 

 little share in their support. Dentition similar in both jaws. 

 Teeth at the symphyses numerous, small, and conical, furnished 

 with three' to five cusps in the young ; those behind broad and 

 pad-like, arranged in oblique rows, the teeth forming the tvFO 

 middle rows being much larger than those in the front or behind. 

 Living species, oviparous. Egg-cases large, with an external 

 spiral lamina (Fig. 245). 



About four species belonging to one genus, Heterodontus 

 ( = Cestracio7i) (Fig. 253), or possibly to two, represent this 

 dwindling family. All are inhabitants of the Pacific Ocean (J'apan, 

 Amboyna, Australia, the Galapagos, and the Californian coast of 

 North America). Little is known of their habits. They feed 



