Recent Literature. 233 
possible, it supports the conjecture that these teeth in the two groups 
are neither homologous nor homodynamous, although bearing a 
superficial analogy.! 
) regards homology, in all the Quaternary and recent Dip- 
rodonts it is the median incisor which is hypertrophied ; whereas, 
in the Mesozoic genera—in which the mode of reduction has been 
observed—the second incisor, or, rather, one of the lateral incisors 
is hypertrophied. In the Bolodontide, as demonstrated by Marsh 
in his observations upon Allodon, the median incisor is reduced and 
the second incisor is hypertrophied. In Tritylodont there are two 
incisors : the outermost is close to the maxillary suture; the hyper- 
trophied incisor is close in front of this and widely separate from 
its opposite fellow, indicating that if this genus is descended froma 
form with three or four incisors (asa comparison with Bolodon ren- 
ders probable), it is again the median incisor which has disappeared. 
Ve have no further evidence bearing upon this point, so it will be 
of importance to observe which of the incisors is hypertrophied in 
Plagiaulax? 
Plagiaulacide. Professor Marsh recently called my attention to 
the wearing of the posterior face of the large upper incisor of Allo- 
‘don by the tip of the lower tooth. This interference forces the jaw 
wards as it ascends by a mechanism similar to that in the ro- 
dents, as demonstrated by Cope’ It has effected a rapid reduction 
of the other incisors, as witnessed in Tritylodon and on; a 
total reduction, as witnessed in Polymastodon and Plagiaulax. In 
contrast with these genera, the recent Diprotodonts present, for the 
Most part, three upper incisors ; while the reduction of all, except 
$° median incisors and the fore-and-aft grinding motion, is con- 
to a single family, the Phascolomyide. 
a 3.) The longitudinal arrangement of the conical tubercles in two 
c more rows is peculiar to the Multituberculates, and, according to 
ope, forms a sufficiently clear sub-ordinal distinction.’ : 
The technical question of taxonomic position is, however, of minor 
! In the Postseri . 
s pt, p. xv., bv a slight oversight, the author refers to 
with Cerium, instead of Siyticion ta having been compa Osborn 
* Mr ras sochloris, (See Osborn, Proc. Phila. Acad, 1887. 
two inci ydekker mentions (p. 195, footnote) that Lemoine deseribes 
‘Not come a Aheloy sn in the magillary series of Plagiaulax. I have 
3 is description. 3 
ue oe echanical Causes of the Origin of the Dentition of the Roden- 
phe pcan Naturalist, January, 1888, p; 12. 
ertiary Marsupialia. American Naturalist, 1884, p. 188. 
