The Embryology of Heterodontus japonicus 661 



Text-figure 3) shows no more than a faint suggestion of this humpbacked appearance. The 

 hump is only slightly developed in the adult Heterodontus phillipi photographed (from 

 life) by Saville-Kent (my Text-figure 4). In one of my specimens of H. francisci, the hump 

 is so low as to be scarcely noticeable. On the basis of all the available data, one can scarce' 

 ly say that the hump is typical for the genus Heterodontus. It occurs in at least four 

 species, but is decidedly variable. In those individuals in which the hump is well de- 

 veloped, the head and "shoulders" have a profile mildly suggestive of a buffalo bull. This 

 resemblance may be partly responsible for the name "Bullhead Shark." 



FAMILY AND GENERIC CHARACTERS 



In this section we are concerned with the distinctive characters common to those 

 representatives of the family Heterodontidae that have survived to the present time. 

 Since all recent species belong to one genus, Heterodontus, the distinction between family 

 and generic characters is, for our purpose, of little importance. In the family Hetero- 

 dontidae, Bridge (1904) includes at least five other genera that are known only as fossils; 

 nevertheless, his brief description constitutes an excellent introduction to the study of 

 living Heterodontids. Some points in the following quotation (from Bridge, 1904, p. 444) 

 are illustrated by references, in square brackets, to figures in the present article. 



Family 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 [Text-figures 3, 4 and 5] . Trunk 

 thickset and almost trihedral, covered with fine shagreen. Nostrils ventral but nearly termi- 

 nal, with oronasal grooves [Text-figures 25 and 40, pages 692 and 711]- Spiracles small, beneath 

 the eyes [Text-figures 3 and 4] . Two dorsal fins, each with a spine in front, the first opposite 

 the interval between the pectorals and the pelvics, the second in front of the anal. Vertebral 

 centra asterospondylic when fully developed. Palatoquadrate cartilages with an extensive 

 articulation with the sides of the preorbital regions of the cranium [Text-figure 33, page 700], 

 the normal suspensoria of a hyostylic skull (hyomandibular cartilages) taking little share in 

 their support. Dentition similar in both jaws [Text-figures 11 and 14c, pages 671 and 673]. 

 Teeth at the symphysis numerous, small and conical, furnished with three to five cusps in the 

 young; those behind broad and padlike, 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 [Text-figure 37, page 706; and Figures 76 to 

 78, plate VII]. 



Continuing, Bridge notes that all the living representatives of this family are in- 

 habitants of the Pacific Ocean, and that they feed principally on molluscs, the shells of 

 which are crushed by their massive grinding teeth. According to Bridge, the different 

 species vary in size (length) from two to five feet. 



Some additional characters of the family Heterodontidae (Cestraciontidae) are listed 

 by Goodrich (1909) as follows: The base of the pectoral fin grows forward below the 

 last three branchial slits (my Text-figure 6, page 666). The pectoral girdle is very powerful 

 (see also Daniel, 1915, Fig. 6, pi. III). According to Goodrich the suspension of the 



