OF THE MESOZOIC MAMMALIA. 
241 
of the fang preceded the division of the crown.' Starting then with the assumption, ^ 
which the Dromotherium dentition seems to support, that the primitive mammals had 
monocuspid molars with incompletely divided fangs, we observe four distinct lines of 
modification in progress in these pre-cretaceous mammals ; these are partly in 
the nature of progression from the reptilian condition and partly in the 
acquisition of changes of form leading directly towards the modern mammalian type 
of molar. 1 The division of the fang, followed in some cases by a rotation of one of the 
fangs fiom a fore and aft to a transverse position with relation to the other and a 
further subdivision. 2° The development of the internal cingulum. 3° a The growth 
of anterior and posterior cusps upon the faces of the primitive cusp, h, the rotation in- 
wards of the lateral cusps to form a triangular crown. 4° and 5° The growth of in- 
ternal cusps from the internal cingulum to form a crown with transversely opposed cusps. 
1. ° The first stage of the evolution of the fang is seen in the Triassic genera. 
The second stage, in which the fangs are entirely distinct and in the same fore and aft 
line, is seen in all the lower Jurassic (Stonesfield slate) genera. The upper Jurassic gen- 
era present three types of modification of the molar fangs. In the Triconodontidce 
and AmpJiitheriidcB the fangs are in the same fore and aft line, conforming to the sim- 
ple condition of the crowns. In the Kurtodontidce, so far as it is possible to deter- 
mine their relations, the fangs are placed opposite each other, i. e., transversely, and are 
somewhat connate ; possibly the crown is passing into a prismatic condition, with a 
single pulp cavity ; but this inference must be taken with reservation. In the Sty- 
lacodontid CP. the fangs are also opposite ; in the lower molars there is but a single fang 
seen upon the outer face beneath the protocone ; on the inner face, however, there are 
two fangs visible beneath the para and metacones ; it is a question, whether one of the 
latter may be connate with the outer fang^ ; if not, these molars are three-fanged, and 
have in this respect already acquired the higher mammalian condition. 
2. ° The internal cingidum, as already observed, is wanting upon the Droinothe- 
rkim molars, but is, possibly, present in Microconodon. As a general law, the inter- 
nal cingulum is present upon the molars of all the Jurassic genera in which the cusps 
are not transversely opposed, and absent in molars in which the cusps are thus opposed. 
Examples of these two types are seen among the Triconodontidce, in which the cingu- 
lum is most strongly developed, and the Stylacodontidw. This law, which may find 
' This point ha.® been ably discussed by Wortman, “ Comp. Anat. of the Teeth of the Vertebrata.” Am. 
Sys. of Dentistry, 1886, p. 420. 
2 Oldfield Thomas, “On the Homologies and Succession of the Teeth in the Dasyuridse,” Phil. Trans. 
1887, p. 456, is inclined to adopt the Baume hypothesis, that in their first stage, mammalian teeth are simple 
cones, rootless for part if not the whole of the animal’s life. 
SI have not been able to examine these fangs very minutely. Marsh describes these molars as bifanged : 
“ Seen from the outside, these teeth appear to be inserted by a single fang, but, in most cases, each has two 
roots, although these are nearly or quite connate.” Amer. ,Tour. Sc., April, 1887, p. 335. Several of the speci- 
mens' seem to present two well-separated internal fangs. If either is connate with the external fang, it is prob- 
ably the most anterior, since the posterior would probably be developed beneath the third cusp, or heel. 
