A Milk Dentition in Oiycteropus. 



247 



which it resembles closely in size ; on the other hand, however, its com- 

 plete calcification is in marked contrast to the soft condition of the 

 other permanent teeth, and therefore it seems safer for the present 

 to call it a backward milk tooth rather than a precocious permanent 

 one. This question again will be easily settled when rather older 

 specimens are available for examination. 



In front of the three posterior teeth there are normally four 

 very minute styliform ones, similar to, and equidistant from, each 

 other, the most anterior placed close to the premaxillo-maxillary 

 suture. Their shape is as shown in fig. 3. On one side of one of the 

 specimens, however, there is an additional minute tooth near the 

 suture, so that there are eight, instead of seven, milk teeth present in 

 the jaw. 



Below, the dentition appears to be in a rather more advanced state 

 of development, so that in the larger specimen the germs of the per- 

 manent teeth are distinguishable as well as the milk teeth (PL 8, 

 fig. 2). The latter are here apparently only four in number; the 

 posterior one, as in the upper jaw, is large and two rooted, and is 

 placed directly over the germ of what appears to be the fourth tooth 

 from the back in the adult animal. The three teeth in front of this 

 large one are minute, pointed, about equidistant from one another, 

 and apparently placed in relation to the permanent teeth as is shown 

 in the drawing. Between the two most anterior of these teeth there 

 is a larger one, equally elevated in the jaw with them, but as yet 

 quite uncalcified, and therefore no doubt merely the tip of one of the 

 small anterior permanent teeth. 



As to the structure of the milk teeth, a horizontal section of the 

 last upper one, ground down in the dry state, presents the appearance 

 shown in figs. 5 and 6. The numerous large openings seen in the 

 sections are obviously the sockets into which pulp-papillas have 

 extended, and it is evident that if further material were available, 

 and the teeth were properly prepared and cut into sections, a com- 

 mencement of the remarkable histological structure characteristic 

 of the permanent teeth would be found in the earlier dentition. 



Since then it appears that the three large posterior teeth of 

 Orycteropus, already distinguished by their more molariform shape, 

 do not have milk predecessors, while all the small teeth anterior 

 to them do, and in addition the last milk tooth is markedly 

 different from those in front of it, we ought apparently no longer to 

 look upon this animal as homodont, but instead to consider it as 

 an originally heterodont form in which the incisors and canines have 

 been suppressed to allow free play to the mobile vermiform tongue. 



But important as a knowledge of the presence of a milk dentition 

 in Orycteropus is, it does not at present render any easier the difficult 

 questions as to the phylogeny and systematic position of that animal. 



