STRUCTURE OF THE ENAMEL IN THE PRIMATES. 605 



had been derived from a Multituberculate, a Marsupial, an 

 Insectivore, a Lemur, or whether the tooth was that of a Jerboa ; 

 and further, as the evidence adduced in this paper shows, in the 

 case of a Lemur one could differentiate between the Mascarene 

 and the Asian and African forms. 



It remains for me to give a very brief description in general of 

 the tubular enamels of those mammals apart from the Lemurs to 

 which I have alluded. 



The enamel pattern of the Multituberculates is quite distinctive, 

 and differs fundamentally from all others which I have examined. 

 If a section of a tooth of Polymastodon be ground and examined 

 (PI. VII. fig. 5), tubes {t) will be seen passing from. the dentine 

 {d) into the enamel (e) : by careful illumination a faint pattern 

 may be discerned, which can be developed by washing the section 

 for a time in acid alcohol until such an image as is shown in 

 PI. YII. figs. 2, 3, and 4 is disclosed. 



Here one sees a series of horseshoe-shaped bodies (A.s.), which 

 become smaller and more widely separated as the outer edge of 

 the enamel is approached. In A^diatever plane the section may be 

 cut these structui^es never become complete circles, the two ends 

 remaining apart and terminating in bulbous slightly recurved 

 enlargements. Such an image would be seen in tiansverse 

 section of any spiral structure. 



Any further description of the minute structure of this peculiar 

 enamel would be ovit of place in this paper, but it may be men- 

 tioned that Ptilodus also has an enamel which is richly tubular 

 and an enamel pattern similar to, but by no means identical with, 

 that of Polymastodon. 



The Marsupials possess a very distinctive general pattern, as is 

 shown in PI. VIL fig. 1, where the rods or prisms {p) are seen, 

 in transvei'se section, to be arranged in rows which sometimes 

 merge : these rows are separated by a very definite area of inter- 

 prismatic material {i.pjn.}, so that, as Dr. Mummery has pointed 

 out, in teased preparations of developing enamel this substance 

 splits up into laminae. The dark dots (t) are the so-called tubes 

 seen in section. A section showing the enamel prisms in such 

 rows and presenting the tube-penetration is peculiar to 

 Marsupials. 



I have not figured the richly tubular enamal of the Jerboa, but 

 it is the only rodent which shows any trace of tube-penetration, 

 and its enamel pattern presents the criss-cross arrangement of 

 the rods which Sir John Tomes showed to be a character peculiar 

 to the Rodents. 



With regard to the significance of the presence of a system of 

 tubes in the enamel, the variability of its degree and of its 

 distribution and the fact that, Avbilst richly developed in one 

 creatui'e it may be totally absent in a closely-related form, would 

 seem to indicate that it is connected with some adaptive process, 

 and that, taken alone, it has little or no value as evidence of 

 affinity or line of descent. 



