580 ME. M. r. wooDWAiiD ON [May 5, 



tooth ; at the horns of the crescents, t. e. at the anterior and 

 posterior extremities of the outer border, and in the middle of 

 this edge wliere fke two crescents meet, sliglit additional cones are 

 raised up ; these, however, appear very late (Plate XXVI. fig. 35, 

 a & b, 1, 2, 3, i). 



Though no less than four stages were examined, yet it was not 

 quite possible to determine which cusp was the first to appear, 

 for even in the earliest stage of m. 3 two slight prominences were 

 already visible corresponding to the paracone and metacone. In 

 the case of m- 1 and m- 2, three cusps were present in all stages, 

 but of these the two external were alone conspicuous in the 

 younger stages, the antero-external (paracone) being the largest, 

 though in the adult it is smaller than the metacone ; this, I think, 

 shows that the paracone is the first to develop. The internal pro- 

 tocone (7) appears late as a low inward extension from tlie base of 

 the paracone (fig. 32) and cannot possibly be regarded as the original 

 axis of the tooth. The 4th cusp to appear is the small anterior 

 external cusp, which is connected with the anterior slope of the 

 paracone, the hypocone evidently appearing very late. 



In the lower molars the protoconid forms the main axis of the 

 dentine germ, and develops long before any of the other cusps, the 

 next in order being the nietaconid, followed by the hypoconid and 

 entoconid, and lastly the paraeonid. The heel itself minus its two 

 cusps is developed very early before even the metaconid. The 

 paraeonid is especially lute in its development; consequently 

 the molar tooth before this cusp appears presents a very curious 

 shape, the entire antero-external region of the dentine germ being 

 absent. 



Upper 'molars. Loiver molars. 



1. Paracone. 1. Protoconid. 



2. Metacone. 2. Metaconid. 



3. Protocone. 3. Hypoconid. 



4. Small antero-external. 4. Entocouid. 



5. Hypocone. 5. Paraeonid. 



Genbbal Compabision or Eesults. 

 !ZVi« 4ih Premolar, 



The homology of the 4th premolar of the Placentalia with the 

 posterior premolar of the Maraupialia was first ])oiuted out by 

 'J'homas, and there can be no doubt that this tooth in the two 

 groups presents certain constant and striking features ; thus dpm. 4 

 is nearly always molariform, whereas ppm. 4 is often almost unique 

 in its pattern, being a highly specialized tooth, which in those 

 cases where it resembles any other tooth has a striking similarity 

 to dpm. 3 (Hypsij/rymnus, Ganis, &c.). 



Some time ago I pointed out that iu Macropus the so-called 

 ppm. 4 developed from the dental lamina between dpm. 3 and 

 dpm. 4 (28, pi. 36, fig. 19), and was evidently serially homologous 



