OF THE MESOZOIC MAMMALIA. 
255 
The longitudinal arrangement of the conical tubercles in two or more rows upon 
the molars has no parallel among the Diprotodonts. The only approach to it is in 
Myrmecohius. The most striking dental feature of both these groups is the hyper- 
trophy of a pair of incisors in each jaw ; but so far as a close comparison of these 
incisors in the fossil and recent forms is possible, it shows that these teeth in the two 
groups are neither homologous nor homodynamous, although they bear a superficial 
analogy. As regards homology ; in all the quaternary and recent Diprotodonts it is 
the median incisor which is hypertrophied, whereas in the mesozoic genera, upon 
grounds given above, the second incisor, or rather one of the lateral incisors was hyper- 
trophied, while the median incisor was atrophied or suppressed. As regards homo- 
dynamy ; the nearly universal characteristic among the multituberculates of a fore 
and aft grinding motion between the alternating rows of tubercles, was associated with 
the rapid reduction of the upper and lower incisors to one pair. In contrast with this 
disposition, the recent Diprotodonts present, for the most part, three upper incisors ; 
while the extreme reduction and fore and aft grinding motion, are confined to a 
single family, the PhascolomyidcB, in which alone the incisor function is like that in 
the Kodents. 
The relationship to the Monotremes is possible. As observed by Cope, Poulton’s 
description ' of the true teeth in the young Ornithorhynchus paradoxus at once reminds 
us of the dentition of Plagiaulax. “ The anterior tooth of the upper jaw was long, 
narrow and simple as compared with the others ; it was very fully developed, contain- 
ing completely formed dentine and enamel, and its apex was nearly in contact with 
the lower surface of the oral epthelium. All the other teeth were broad and large, 
those of the upper jaw possessing two chief cusps on the inner side of the crown, and 
three or four small cusps on the outer side, while this arrangement was reversed in 
the lower jaw.” According to this, the two chief cusps are upon the outer side of the 
lower jaw, whereas in the Plagiaulax series (fig. 7, text) they are invariably upon 
the inner side ; this fact lessens the degree of resemblance, but there is little question 
that these teeth are of the rare multituberculate type, and this discovery has an impor- 
tant bearing upon the problem. The humerus of Ornithorhynchus and Echidna pre- 
sents a single convexity for the radius and ulna, the proximal face of the radius being 
placed immediately in front of the ulna. The multituberculate humerus {PoJymasio- 
don, 3Ienisco^sus) presented a double convexity; the ulna and radius were placed 
transversely. In some Ornithorhynchus specimens there is a stout intertrochlear 
ridge, as in the above multituberculates. 
Such comparisons leave no certain result. The separation of these genera from 
the Diprotodonts, justifies the prediction, as a result of future discovery, that the 
Multituberculata will prove to be the last representatives of a very ancient phylum 
I Proc. Roy. Society, Feb. 9th, 1888, p. 353. Abstract in Nature, Feb. 16th, 1888, p. 383. See Appendix. 
