32 BULLETIN 57, UNITED STATES NATIONAL MUSEUM. 
cone to parastyle. A similar commissure occasionally extends pos- 
teriorly behind base of metacone to metastyle, but this is less con- 
stantly present. The paracone and metacone form the highest 
portion of the tooth ; the metacone, together with third and fourth 
commissures, is usually larger than the anterior cusp and ridges. 
A faintly developed cingulum may usually be traced along anterior, 
inner, and posterior edges of crown. On outer side the cingulum is 
practically absent. The third molar is always smaller than either of 
the others, and some of its elements are reduced or absent. The 
process of reduction, which invariably proceeds from behind forward, 
varies, in teeth that can not be regarded as abnormal, from a mere 
shortening of the metastyle and fourth commissure, accompanied 
by diminution in height and diameter of the metacone, as seen in cer- 
tain Ehinolophidae (Plate I, fig. 1), to absence of everything except 
the parastyle, paracone, protocone, the first commissure, and a trace 
of the second, elements which are always present in this tooth. This 
condition is well illustrated by Pachyotus (Plate I, fig. 2). 
In the mandible the first and second molars are also alike in form, 
with the second usually a little the larger. Five cusps are present, 
the outer anterior protoconid (fig. 4, jircd.) , the inner anterior para- 
conid (fig. 4 , pcd.), the inner median metaconid (fig. 4, mcd.), the 
outer posterior l^poconid (fig. 4, hcd .) , and the inner posterior 
entoconid (fig. 4, ecd.). In general appearance these teeth resemble 
the outer higher portion of the upper molars reversed, the three 
inner cusps corresponding in form to the styles, and the protoconid 
and hypoconid to the paracone and metacone. The hypoconid is, 
however, lower than the protoconid, though the width of the tooth 
through the base of this posterior cusp is normally greater than that 
through the protoconid. The points of the cusps of the two rows 
are nearer together than in the upper teeth, and the commissures 
are shorter and more strongly concave, the third extending down- 
ward from point of hypoconid to base of metaconid. The two seg- 
ments of the W are therefore disconnected, though this is not at 
first sight apparent when crowns are viewed from above. A well 
developed though not very prominent cingulum extends around outer 
base of crown from paraconid to entoconid. Third lower molar 
with posterior segment usually much smaller than the first, owing 
to the great reduction in size of both hypoconid and entoconid and 
the close approximation of these two cusps. In some Rhinolophida? 
and Emballonuridae, however, the tooth is practically identical with 
m x and m 2 (Plate II, fig. 1). In the first and second upper molars 
of insectivorous bats the metacone .may be said to be the dominant 
cusp. It is not only the largest and the first to appear above 
level of alveolus as the teeth grow, but in the modifications to 
which the form of the crown is subjected the metacone invariably 
