702 Bashford Dean Memorial Volume 



It is apparent that paleontologists have experienced considerable difficulty in 

 disentangling the Cestraciontidae frora the Hybodontidae. The two families have, at 

 least once, been lumped together, and authors have seldom agreed on the criteria by means 

 of which they should be divided. Wherever the line has been drawn, the distinction 

 seems more or less arbitrary : the differences between the families seem no more impressive 

 than the differences between genera w^ithin at least one of the families. These facts 

 cannot be wholly explained on the ground of difficulty in reading the paleontological 

 record; for nearly complete skeletons belonging to several different genera have been 

 obtained. The only adequate explanation is that there exists a close genetic relationship 

 between the famiHes. With respect to famiHes other than the Hybodontidae, the Het' 

 erodontidae occupy a relatively isolated position. Woodward (1921) states that the 

 Hybodonts are a generalised group from which several later famiHes appear to have 

 risen. They were the dominant sharks of the Jurassic and Early Cretaceous Periods. To 

 the present writer it seems not only possible but highly probable that the Mesozoic 

 Hyhodus, or some Hybodont closely related to it, is the direct ancestor of Heterodontus. 

 After this glimpse into the past, we return to the study of Hving Heterodonid sharks. 



SEXU.^ DIMORPHISM AND THE REPRODUCTIVE ORGANS 



Concerning the Port Jackson Shark, Heterodontus phillipi, Maclay and Macleay 

 (1879) state that the two sexes scarcely differ in size and marking. With the aid of special 

 drawings, they describe the intromittent organs (myxopterygia or "claspers") of the male 

 H. phillipi. More recently, the claspers of three species of Heterodontus (philUpi, japonicus 

 and galeatus) have been described and figured by Leigh-Sharpe (1922 and 1926). Some 

 marked specific differences in this organ are noted. 



According to Dean's notes, Heterodontus japonicus show^s marked sexual dimorphism. 

 The female is larger than the male, heavier in body and somewhat different in proportions. 

 Dean states that the female, when full-growm, measures about 1200 mm. (47 inches) in 

 total length: the male, about 1000 mm. (39 inches). There is little difference in color, 

 though Dean at one time befieved that the males could invariably be distinguished, in the 

 w^ell of a fishing boat, by a darker and richer tone. 



Since I have no adult female specimen of H. japonicus available for dissection, it is 

 a satisfaction to be able to record the results of my examination of the reproductive organs 

 of the larger female specimen of H. francisci belonging to the American Museum of 

 Natural History. This shark is 705 mm. (27-7 inches) long, and is fully adult. The oviducts 

 of both sides of the body are well developed, with especially large, thick-walled shell 

 glands. Evidently both oviducts are functional. As in the adults of most sharks, the 

 two oviducts have a common abdominal aperture. In decided contrast to the oviducts, 

 the ovaries of the two sides of the body are very unequally developed. 



On the right side, the large ovary contains eggs in various stages of development. Of 

 these, the two largest measure about 35 rmn. in diameter, the next largest one about 



