Prof. Owen on Dicynodon tigriceps. 239 



exaggerated by the less entire state of their bony circumference in the specimen of 

 the first-described and smaller reptile. 



In a comparison of the occipital region, the mastoideo-parietal ridges appear in 

 D. tigriceps to subside sooner as they ascend, converging to the top of that region ; 

 and, though these ridges are less entire in the larger than in the smaller skull 

 compared, the difference does not seem to be wholly due to accidental mutilation : 

 the part above the foramen magnum seems to be naturally more concave from side 

 to side than it is in D. tigriceps, in which the above ridges meet at top and there 

 overhang the superoccipital region. In D. lacerticeps, the par- and ex-occipital 

 plates abut against the mastoids and tympanies by three short thick truncate pro- 

 cesses on each side : in D. tigriceps the union of the same parts seems to have 

 been continuous and unbroken. The hypapophyses are relatively shorter and 

 thicker in D. lacerticeps. The whole occipital region is broader in proportion to 

 its height in D. tigriceps. 



In a comparison of the upper surface, the cranium proper is shorter in propor- 

 tion to its breadth, and the foramen parietale is relatively nearer the back part in 

 D. tigriceps; and the flat space between the temporal ridges is relatively much 

 greater. In D. lacerticeps (see PI. III. fig. 2), these ridges nearly meet behind the 

 foramen parietale. This is an important difference, as bearing on the question of 

 the relation of the smaller to the larger skull, in regard to individual age. A young 

 carnivorous animal differs from an old one of the same species in the smaller 

 extent to which the origins of the temporal muscles ascend on the sides of the 

 cranium ; the interspace above, between those ridges, being broader as the animal 

 is younger ; whilst in the full-grown, especially the males, it becomes obliterated, 

 and a further extent of origin is afforded to the biting muscles by the development 

 of a median parietal crest. 



The extent of origin of the temporal muscles from the sides of the parietal bone, 

 in D. lacerticeps, indicates the small skull, exhibiting that extent, to have been of a 

 full-grown, if not old individual ; the smaller relative extent of the same muscular 

 attachments, and the wider flat interspace between those origins, in D. tigriceps, as 

 shown in the two large equal-sized skulls, which seem to have come from adult 

 individuals, plainly indicate, with the different form and proportions of the cranium, 

 and the other differences above pointed out, a specific distinction from the D. 

 lacerticeps. 



The profile of the skull in D. lacerticeps begins to slope or curve down from a 

 line parallel with the back part of the orbits ; but in the great D. tigriceps it does 

 not begin to bend down until in advance of the orbits ; and the preorbital part of 

 the skull, with the interorbital space and the interspace of the nostrils, is relatively 

 broader in D. tigriceps than in D. lacerticeps. 



VOL. VII. SECOND SERIES. . 2 K 



