OF THE MESOZOIC MAMMALIA. 
249 
dee retain four inciaon*. the first and second premokrs are atrophied, or the first sup- 
pressed, and the atrophy of the molars is at both ends of the series. 
The only instance in which a vertical replacement of the teeth has been observed 
IS in the genus T,-iconodon, fully described upon page 198 of this memoir, which suc- 
cession IS confined to the last premolar, precisely as in the modern marsupials. Pria- 
codon may prove to present a similar mode of succession. 
General Conclusions. 
1. The primitive mammalia, ancestral to the known Jurassic mammals, were 
heterodont. The teeth were without diastema and divided into three series, incisors, 
premolars and molars. The incisors were separate and inserted by a single fang. 
The premolars had single or grooved fangs and simple conical crowns ; the addition 
of cusps took place at the base of the crown in connection with the internal cingu- 
lum ; first, by the addition of a heel and its elevation into a posterior cusp ; second, 
by the addition of an anterior basal cusp. The molars had grooved fangs and simple 
conical crowns ; the additional cusps were found upon the anterior and posterior 
slopes of the crown above the base, or upon a heel from which secondary cusps arose 
as in the premolars ; or the development of secondary cusps was wholly at the base of 
the crown. Canines were difi’erentiated from the first member of the premolar series, 
and at first were distinguished by their larger size from the first true premolar, sub- 
sequently by the coalescence of the paired into single fangs. 
2. Tlie complication of the molar crowns increased from before backwards, and con- 
versely the ju'imary division of the fangs probably took place from behind forwards, first 
in the molars, then in the premolars but not extending to the incisors. Rotation of one 
of the fangs inwards and triple division of the fangs, accompanied the development of 
internal molar cusps. 
3. Tlie typical dental formula was i4, cl, p4, m8. Reduction of this formula 
was effected by the loss of the lateral incisors, resulting possibly from the hypertrophy 
of the adjoining canine; the premolars were reduced by regular antero-posterior 
suppression {P/iasr.olotherium'l), or by the loss of the first or second member of the 
series ; molars were reduced either by antero-posterior or by postero-anterior reduction 
or by simultaneous reduction of both ends of the series. 
4. ’ The complication of the molar crowns and specialization of the dental series 
into the four sharply defined groups, incisors, canines, premolars and molars took 
place independently of reduction, i. e., some of the genera in which the dental groups 
were most sharply defined, retained the typical formula. The specialization of the 
incisors and canines for different functions, in different genera, proceeded with com- 
parative rapidity. The premolars were the most conservative members of the series, 
retaining longest the primitive common pattern. The molar differentiation proceeded 
