452 MEMOIRS OF THE CARNEGIE MUSEUM. 



the median and prefossettes, the walls of the internal face of the ectoloph and the 

 external face of the crochet are rapidly slanting toward one another, so that on 

 extreme wear the tooth would have the usual api:)carance seen in old individuals 

 of this species. The post-fossette is deep and broad. The cingulum is less de- 

 veloped internally than on the milk premolars. M" is just appearing in its alveolus 

 and M' is entirely buried in the maxillary. 



2. In a somewhat younger individual (No. 2476) it is observed that the 

 roots of D.P- are longer and heavier and in excavating the maxillary, P- 

 is found in an extremely early stage of formation (often no evidence of it 

 is found). M' in this individual is just cutting through the alveolar border, 

 while that in the specimen described above has received slight wear. There 

 can be only a comparatively small difference in age of these two individuals, and 

 it thus appears that the permanent teeth developed extremely rai^idly after they 

 began to show the form of tooth in the maxillary bone. This rapid formation 

 and development of the permanent dentition in Diceratherium should not be 

 regarded as out of the ordinary when comparison is made with the shedding of the 

 deciduous and the appearance of the permanent teeth in man and other mammals. 



3. In the collection of the Carnegie Museum are two left rami (Nos. 1820 

 and 1821) representing very young animals, most probably foetal. The total 

 length of the rami of each of these young specimens is approximately 180 mm., 

 while the depth in the middle antero-posterior region is 28 mm. The most char- 

 acteristic features are as follows: 



The lunate-shaped outline of the ramus due to the greatly downward curved 

 under border of the jaw in the fore-and-aft direction, the close proximity of the 

 cheek-teeth to the canine and the incisors, due to the absence of a diastema on 

 the alveolar border of the jaw, the very slight constriction in front of the cheek- 

 teeth and back of the incisors which is so very pronounced in adult and even in 

 quite young specimens, the small transverse diameter of the symphysis, and the 

 deep groove on the external face of the jaw extending from the symphysis about 

 20 mm. back in a parallel line with the long axis of the ramus. The glenoid condyle 

 is not present in either one of the rami; the coronoid process on the other hand is 

 present in No. 1820. The latter is rather low, and terminates in an attenuated 

 process. 



The second deciduous incisor is in place, while the alveolus for the first is 

 empty. The lateral incisor is not present and a small opening external to D.I2 

 of this individual may or may not have contained this tooth. The small alveolus 

 for the canine is immediately in front of that for D.Pi; the latter is a round opening 



