26 DISCOVERY REPORTS 



separated from each other only by a suture. The lateral portion of the palatine is smaller in H. planifrofis 

 than in H. rostrattis, and in general the impression obtained is that in the former species the pterygoid 

 anteriorly has expanded at the expense of the bones adjacent to it. 



The pterygoids are of typical ziphoid form in both species, ' large, solid, backwardly produced, 

 meeting in the middle line, not involuted but simply hollowed on the outer surface' (Flower, 1871). 



The zygomatic process of the malar has its origin much nearer the posterior border of the bone in 

 H. planifroiis than in H. rostratm, in which species it originates only a little way behind the antorbitai 

 notch. Differences in the anterior margin of the antorbitai region involve the malar bone and were 

 referred to in the description of the dorsal view of the skull. 



The lachrymal, a distinct bone, has the same essential form in both species. The extent to which 

 it appears on the lateral border of the skull has already been referred to. It is long and narrow, 

 extending obliquely backwards from the external margin of the skull to the infra-orbital foramen. 

 It is bounded anteriorly by the malar and maxilla and posteriorly by the orbital process of the frontal. 



The external margin of the orbital process of the South Australian specimen is more pronouncedly 

 concave than that of the Discovery specimen. 



No marked differences are discernible in the squamosals either between the South Australian and 

 the Discovery specimens or between either of these and H. rostrattis. 



The tympanic-periotic bones are very similar in H. rostrattis and H. planifrons, and as in the 

 former species so in the latter they are secured to the skull anteriorly by a slender inward-curving 

 process from the squamosal embracing the periotic, and posteriorly by a rugose wedge-shaped 

 extension from the tympanic between the squamosal and basi-occipital. 



In the posterior view of the skull all the available specimens of H. planifrons show the characters 

 to which Flower drew attention in his description of the type, namely, the narrowness and greater 

 height compared with H. rostratus and also the inferior size of the occipital condyles in the southern 

 species. 



MANDIBLE (Fig. i) 



The jaws of H. planifrons compare closely with those of H. rostratus in general shape and in the 

 extent of the symphysial region. The two rami of the mandible are not ankylosed at the symphysis 

 in the Discovery specimen, whereas in the South Australian specimen Hale describes fusion as pro- 

 ceeding, the two rami being linked by ossified bridges. In the former specimen the tooth alveolus at 

 the tip of each ramus is continuous posteriorly with the dental groove, gradually merging into it. 

 In the South Australian specimen (Hale's Fig. 4) the alveolus appears to be sharply defined from the 

 dental groove. 



These differences between the jaws of the two specimens are such as might be expected from their 

 difference in age. 



TEETH (Fig. 4) 



The outlines of the teeth of the Discovery specimen and of the South Australian specimen show the 

 main differences between the two. Those of the former are conical and slender, and have a widely 

 open pulp cavity. The dimensions are as follows : 



Right Left 



(i) Length 50 mm. 50 mm. 



(2) Greatest diameter 18 mm. 18 mm. 



(3) Diameter at right angles to (2) 17 mm. 16 mm. 



