450 MEMOIRS OF THE CARNEGIE MUSEUM. 



fact is the great number of bones representing young animals and females in pro- 

 portion to those of males. This would appear to indicate (1) that the animals 

 were polygamous to a great degree and that the males were either struggling for 

 the possession of the herds after the manner of recent ungulates (Equus) , and were 

 few, or, that they were strong enough to extricate themselves when overtaken by 

 the calamities which destroyed the herds. 



The articulated skeleton of Diceratherium cooki has been fully discussed in 

 the Annals of the Carnegie Museum, Volume VII, pp. 274-279. 



Modes of De\t:lopment of Certain Dental and Bony Structures of the 



Cranium in Diceratherium. 

 (Plates LXV and LXVI.) 



Important facts, in connection with the evolution of the dental formula and 

 other features of the cranium of the Rhinocerotidse, are obtained from the large 

 collection under study in the Carnegie Museum. Some studies bearing on the 

 evolution of the incisors and canines of Diceratherium were already presented before 

 the Paleontological Society at Pittsburgh in 1910. The following pages are given 

 to a further discussion of the appearance and shedding of the different deciduous 

 teeth, the appearance of the permanent series, and other changes of contour of 

 the head from the young to the fully adult form of Diceratherium cooki. 



1. A skull of a young Diceratherium, No. 1848 (See PI. LXV, Figs. 1, 2, 4) 

 which belongs to the original series from which the type of D. cooki was selected, 

 is especially complete and furnishes an excellent opportunity for study. In viewing 

 this skull from above, the most noticeable characters are the following: brain-case 

 proportionally broad; occiput short; frontals broad; horn-cores little developed, 

 and nasals gradually pointed, more like that of adult females. Back of the horn- 

 cores on the lateral margin of the nasals there is also less constriction in skulls of 

 young individuals and adult females than is the case in males. The supra-orbital 

 ridges are so varied that one cannot attach great importance to them, though it 

 would appear that in female skulls they are generally less prominently developed 

 and in their backward progression to the occiput they possibly have a tendency to 

 be further separated from the median line. On either a direct side view or a palatal 

 view of the young skull the most noticeable feature is the great backward extent of 

 the alveolar border of the maxillary. The alveole for M' is seen to be nearly oppo- 

 site the pterygoid, while in fully adult forms this tooth is well in advance of this 

 region. In very young individuals, the base and the supra-occipital of the skull 

 are often slipped off at the sutures, not an unusual feature of the mammalia. 

 In the skull here described, the base is lost, but the supra-occipital is in position. 



