OF THE MESOZOIC MAMMALIA. 
235 
second prcmolai-. The third premolar is larger with a sub-conical crown. The 
fourth premolar has two fangs which are partly rotated into a transverse position. 
The inner face of the crown is more lofty but otherwise resembles that of the first 
molar. The wearing surface of the molars presents an enamel V upon the anterior 
and posterior faces of the crown, diverging from the inner face, and bisected by a 
faint median ridge, apparently of enamel, which disappears, as it extends outwards. 
SUB-ORDER INSECTIVORA PRIMITIVA.i 
An extinct sub-order which is probably on the line of the primitive Placentalia,. 
with tritubercular molars, forming alternating series in the upper and lower jaws.. 
The nearest affinities in dentition are to some of the recent Insectivora. 
. Insectivorous Sub-Group. — This embraces the families AnMotherikicB and Sty- 
lacodontidoB. The types of the former family are known by the inner surface of the 
mandibular rami only ; and as this is in many respects similar to the corresponding sur- 
face of the Stylacodon types, the separation of these families is not fully established. 
The genera vary from an extremely small to middle size. The smaller genera embrace 
the typical insectivorous forms, in which the incisors are procumbent and spatulate, 
and the canines are very small. In the larger genera the incisors and canines are more 
pointed and erect. In the entire sub-group the premolars lack anterior basal cusps, the 
series inci'eases rapidly in size, the last premolar being lofty and rising much above the 
anterior molar level as in many recent Insectivora. The molars have no internal cin- 
gulum ; the internal face being smooth and tricuspidate. The condyle is very high and 
the coronoid is slender. The angle is slender and produced to a tip posteriorly. The 
mandible is shallow and tapers towards the symphysis. The dental formula departs 
little from the typical pm mg, in the various genera. The adaptation to an insec- 
tivorous diet is very evident in the procumbent incisor-canine series of Stylacodon, 
which indicates the presence of a protrusible tongue. The molars of Dryolestes show 
a striking resemblance to those of the GhrysocMondod'} Another marked feature in 
this sub-group is the retention of four incisors, while in the carnivorous and omnivor- 
ous groups the lateral incisors are wanting in the upper Jurassic genera. It follows, 
that this sub-group is sharply defined from the preceding ones. 
AMBLOTHERIIDA:,'’ Osbom, 1887. 
Molars loith two slender cmps in line and a posterior heel with an external cingu- 
lum, forming an overlapping series ; no opposed, cusps (so far as known). Premolars 
* Provisional ; see Appendix. 
^ St. George Mivart, “ On the Osteology of the Insectivora.” Jour, of Anat. and Phys., vol. II, p. 161. 
It appears from, Mr. Mivart’s description that the lower molars of CalcocJdoris, “with a marked posterior pro- 
cess,” resemble those of Dryolestes even more closely than do tlmse of Chrysochloris. 
= Proc. .'lead., Phila., June. 
