246 



Mr. 0. Thomas. 



"A Milk Dentition in Orycleropus" By OLDFIELD Thomas, 

 Natural History Museum. Communicated by Dr. A. 

 Gunthee, F.R.S. Received December 12, 1889— Read 

 January 9, 1890. 



[Plate 8.] 



Of the few Mammalia in which no trace of a milk dentition has 

 been fonnd, Orycteropus, the Aard-Vark, has always occupied a pro- 

 minent place, owing partly to the pecnliar strnctnre of its prominent 

 teeth, and partly to its very donbtfnl systematic position. 



An opportunity has now fallen in my way of proving that it has 

 after all two sets of teeth, those of the first, or milk set, being rudi- 

 mentary, and probably quite fuuctionless, but nevertheless so far 

 developed as to be all completely calcified, and to be for the most 

 part readily distinguishable by form and position from those of the 

 second or permanent set. 



Among the collections in the Natural History Museum there are 

 two very young females of Orycteropus afer in spirit, presented by 

 Sir Richard Owen, and it is in these that the milk teeth now to be 

 described occur. The larger of the two measures 18 inches in total 

 length, and the smaller 14 inches. 



Each of these specimens has a complete, although rudimentary, set 

 of milk teeth, extending the whole length of the maxillary bones 

 above, and along a rather shorter portion of the mandible below. 

 None, however, are observable in the premaxilla3, or in the correspond- 

 ing anterior part of the mandibles. The teeth are all quite minute, 

 and it is very doubtful whether they would ever have cut the gum. 

 Specimens rather older than those before me are needed to determine 

 this point. 



In the upper jaw there appear to be normally no less than seven 

 milk teeth (PI. 8, fig. 1). Of these the most posterior (figs. 4 — 6) 

 is by far the largest, has a rudimentary crown, and two distinct 

 roots, anterior and posterior. The second, proceeding forwards, is far 

 smaller, and is simple and styliform. The next, the third from the 

 back, is also simple, but is far larger in section, and its base is not 

 closed up in either of the specimens ; on this account there seems to 

 be just a possibility that this particular tooth is not a milk tooth at 

 all, but only the tip of one of the smaller anterior permanent teeth,* 



* These small anterior permanent teeth fall out before the animal is adult, and 

 are absent in the great majority of Museum specimens. There is, however, an 

 immature skull in the Cambridge Museum which shows tbe alveoli for no less than 

 ten teeth above (at least on one side) and eight below, some of the minute styliform 

 teeth belonging to these alveoli being still in position. For the loan of this skull I 

 have to thank Mr. J. W. Clark, director of that Museum. 



