44 FISHES CHAP. 
Scarcely anything is known of the habits of the Notidanidae 
and the Chlamydoselachidae. 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. 
Fic. 252.—Chlamydoselachus anguineus. (From Giinther.) 
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. Nostrils 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 two 
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 
(=Cestracion) (Fig. 253), or possibly to two, represent this 
dwindling family. All are inhabitants of the Pacific Ocean (Japan, 
Amboyna, Australia, the Galapagos, and the Californian coast of 
North America). Little is known of their habits. They feed 
