The Eynhryology of Heterodoyitus japonicus 



701 



and the cranium. In the skull of Hyhodus hasanus (my Text-figure 34), the cranium is 

 rather short, with a relatively large orbit and with short postorbital and rostral regions. 

 The jaws, which are relatively large and massive, are longer than the cranium, so that the 

 hyomandibular suspensorium extends backward, while the upper jaw extends forward as 

 far as the end of the snout. The rami of the mandible, though deep and massive behind, 

 rapidly taper forward and meet in a com- 

 paratively feeble symphysis which does not 

 extend so far forward as the front of the 



Text'figure 34. 



Restoration of the skull of Hyhodus hasanus 



Egerton, a little less than one-half natural size. 



The deeply shaded portion is the orbit. 



cr., cranium; hy., hyomandibular; I., one of the labial cartilages; 



m., lower jaw or mandible; q., quadrate region of the pal- 



atoquadrate. The lettering does not appear on the original. 



After Woodward, 1916, Fig. 3b. 



upper jaw. The palatoquadrate is weak and 

 depressed at its anterior end, but deepens 

 rapidly backward. According to Woodward, 

 it can scarcely have articulated with the postorbital prominence of the cranium. 



According to Huxley (1876) the skull of Heterodontus is the link that connects the 

 primitive amphistylic skull with the ordinary selachian skull, which is hyostylic. Like- 

 wise, Goodrich (1909) wrote: ". . . it is well established that Hyhodus and Synechodus had 

 typical amphistylic skulls, with the palatoquadrate and hyomandibular as in the Notidani- 

 dae and other primitive Elasmobranchs." This view accords with Woodward's observa- 

 tion (1886) that the skull of Hyhodus duhrisiensis is typically amphistylic, and with 

 JaekePs interpretation of the skull of Hyhodus haujfianus (my Text-figure 28); but it 

 does not harmonize with Woodward's later statement (1916) that the pterygoquadrate 

 (palatoquadrate) of Hyhodus hasanus "can scarcely have articulated with the postorbital 

 prominence of the cranium". It seems remarkable that species of the same genus should 

 differ in a manner so important; but if the skull of Hyhodus hasanus really does lack a post- 

 orbital articulation with the cranium, then it is hyostylic and therefore more like the skull 

 of Heterodontus. By the same token, if such divergences can exist within a single genus of 

 Hybodonts, how trivial become the differences between the skulls of any species of the 

 Mesozoic Hyhodus and the present-day Heterodontus! In view of the well-known diiE- 

 culties attending the restoration of the fossil vertebrate remains to life-like attitudes, one 

 suspects that there is a flaw in the data somewhere; but, considering the long lapse of 

 time, the evolution of the skull of Heterodontus from that of any Hybodont does not seem 

 impossible. 



