The Emhryology of Heterodontus japonicus 685 



their external anatomy they differ so much that they are distinguishable at a glance; but 

 I suspect that even a gifted artist could not portray all their subtle and almost intangible 

 differences of contour. 



In both species the head is broader than the body (excluding the paired fins). The 

 region of greatest breadth lies between the first gill'covers. In the large female specimen 

 of fi. quoyi, the greatest breadth equals 22.3 per cent of the total length; in the decidedly 

 small and immature male specimen of the same species, only 16. 1 per cent. In the larger 

 female specimen of H. fra7^cisci, the greatest breadth equals 19.5 per cent of the total 

 length; in the smaller female of the same species, only 17.1 per cent. It is apparent that in 

 both species the breadth of the head, in proportion to total length, is greater in the older 

 specimen ; but when allowance is made for age (ignoring sex as a possible factor) H. quoyi 

 is definitely broader than H. francisci. For further information we must have recourse to 

 published drawings, which are not so satisfactory as specimens since we have no assurance 

 that they were made from accurate measurements. There are no drawings of either 

 dorsal or ventral views of H. quoyi. In Maclay and Macleay's dorsal view (1879) of 

 their 708'mm. specimen of H. francisci, the greatest breadth (which is in the region of the 

 first gill'Covers) equals 17-5 per cent of the total length — a proportion somewhat smaller 

 than that obtained for my larger specimen of H. francisci, which has almost exactly 

 the same length. 



It may be of interest to extend this comparison to other species, but there we must 

 depend entirely on drawings which may not be made to scale. Maclay and Macleay's 

 dorsal view of a fuU'grown specimen of H. phillipi (my Text'figure 7, page 667) has 

 a head that appears broad as compared with most sharks, but is decidedly narrower than 

 the heads of my adult specimens of H. quoyi and H. francisci. Maclay and Macleay's very 

 young specimen of H. phillipi (my Text-figure 9, page 669) has a head that is much 

 narrower than that of their adult of the same species. In a young female specimen of H. 

 zehra described and figured by Maclay and Macleay (1886) the width of the head cannot 

 be measured because in the dorsal view the head is turned slightly to one side; but it 

 appears very narrow, and the entire body is narrow as compared with other species of 

 Heterodontus. In a drawing by Maclay and Macleay representing a dorsal view of a young 

 female specimen of H. japonicus about 406 mm. long (my Text-figure 24, page 691), the 

 width of the head equals 15.7 per cent of the total length. This is slightly narrower 

 than the head of my young male specimen of H. quoyi, and of course much narrower than 

 the heads of adult specimens of H. quoyi and H. francisci. 



In my adult female specimen of H. quoyi, the height of the head (including the 

 supraorbital ridge) equals 54.2 per cent of its breadth, while in my young male of the same 

 species the proportion is 68.3 per cent. In my larger female specimen of H. francisci 

 the height of the head equals 62.3 per cent of its breadth; in the slightly smaller female 

 of the same species the corresponding percentage is 69.0. We do not know if sex is 

 a factor in determining the si2,e or bodily proportions in these species, so this possibility 

 must be ignored. With this reservation, the data indicate that in the adults of both 



