CYCLOPIDIUS 245 



even more advanced in that its three lobes are all about equally developed. Deciduous premolar 3 roughly corre- 

 sponds to permanent premolar 4. Attention is called to these resemblances, because, when single teeth are found, 

 it is easy to identify a deciduous premolar, as an adult tooth of some smaller species. 



Skeleton: Loomis (1925 A, pp. 244-245) describes the skeleton as follows: 



As noted above, the body of Leptauche?ila or Cyclopidins is small and light in contrast to the skull. Sinclair 

 in his vertebral formula has 14 dorsal and 6 lumbar vertebra?, but my series, which was found intact and articulated 

 from the atlas to the sacrum, shows 13 dorsals and 7 lumbars. There is little about them different from the 

 usual oreodont vertebra;. The ribs, however, are unexpectedly light, and toward the rear change from flat to 

 cylindrical, the posterior ribs being slender rods. Both limbs are of moderate length and extremely light. The 

 radius of the front limb is curved to a greater degree than I have seen in any other oreodont, which with the 

 lightness of the whole leg, seems to indicate great agility. The forefoot, unexpectedly, shows five toes, the first 

 being reduced, but having all the phalanges present. This reminds us of Merycoidodon, and is a primitive char- 

 acter not retained in other Miocene representatives of the phylum. The hind limb offers nothing different from 

 that of typical oreodonts, there being the usual four toes. 



Discussion: Owing to the presence of three lower incisors, Loomis was led to refer this 

 form to Leptauchenia, although he clearly recognized its similarity to the generic characters of 

 Cyclopidius. It has been demonstrated by Schlaikjer and others that Cyclopidius may have three 



Fig. 172. — Cyclofidius densus (Loomis). Manus. HT. Cat. No. 22-595 A.M. 1/2 nat. size. (After Loomis, 1925.) 



inferior incisors but, so far as known at present, always two superior. I am inclined to believe that 

 C. lullianus had three incisors in the lower jaw, but the alveolar parapet is too badly damaged for 

 one to be absolutely certain on this point. 



I quote from Schlaikjer (1935, p. 162): 



It is curious that of the lower incisors I 2 is smaller than Y x or I 3 . It would seem, therefore, that the superior 

 incisor which has been lost is I 2 . Also, if C . brevifacies has but two lower incisors, it is probable that the second 

 rather than either the first or the third has been lost. This is a most unusual character and one which is to be 

 found only in a very few groups of mammals. 



It is entirely possible that the loss of the third incisor may be an old-age character. All of the 

 incisors are very small and at best are not too solidly implanted in the jaw. 



A series of skulls of this species at the Museum of Comparative Zoology at Harvard shows 

 marked variations which are in part due to sex and in part to old age. One of the old-age changes 

 is a tendency toward greater brachycephaly. Of this feature Schlaikjer says: 



Not only in skull proportions is there a change from youth to old age, but the skull in general seems to 

 become heavier in its construction. This is especially true of the malar arches and of the sagittal and lambdoid 

 crests. Likewise, in the dental series marked changes take place with age. For example, each molar tooth is 

 sphenoidal in outline, — that is, toward the base of the crown the anteroposterior dimension diminishes while the 

 transverse dimension increases. Therefore, as the tooth is worn it becomes broader and shorter. 



