248 
ON 
the structure and classification 
, • 1 Prwoodmi by a diastema behind the canine, 
where it is present an is lep ac . ^^known. In the Spalacotheriince 
In the PWIoftmin* the mode of reduction is unknotv 1 
it is •uraiii the «rst premolar which has been suppressed. The mode lediiction ol 
, m* in this family is not certainly known but probably took place from behind 
LLs. This is cerLiy in process in TiWon m winch n, is very much 
retardd in some species, if not actually suppressed. ■ nm 
In the AmfhUheriidm, the mode of incisor reduction is unknoira. e find the 
preniolars reduced at the expense of ft, which is atrophied in D^locyrnd^ and 
Lwon and suppressed in Smeodon. The reduction of the molars ,s obviously 
from behind forwards, Diflayymxh,, having the typical number and EnrKxion falling 
two short of it. . p i j- 
In the PeralestidcB the dental formulae are uncertain. Judging from the diastema 
behind the canine, it would appear that the premolar series of Paurodon has been 
reduced by the loss of pi_2. 
In Kurtodon the typical number of premolars is present but the atrophy of the 
three most anterior, pi_3, is in marked contrast with the hypertrophy of pi- 
General characteristics of the Insectivora Primitiva are the apparently constant 
atrophy of the anterior pair of premolars and hypertrophy of the posterior pair, and 
the reduction in size of the molars at both ends of the series, accompanied by a re- 
tention of all the incisors, and the typical number of molars. 
In the AmhlatheriidcB the first premolar is extremely small and the molar series 
increases regularly in size from before backwards. In the Stylacodontidoe the incisors 
are unreduced, but a slight hypertrophy of the median incisor is almost a constant 
feature, ivith a corresponding atrophy either of the second or fourth incisor. The 
first premolar is atrophied in three genera, so that there is little doubt that this is the 
tooth which is suppressed in Astlienodon, in which there are but three, premolars. 
The second premolar is also small. Adjoining the hypertrophied posterior pair of 
premolars is usually found a very small first molar; the last molar is retarded in 
development and is generally small, so that although there is no case in which either 
of these teeth has been suppressed, we may assume that the molars are in course of 
reduction at both ends of the series. 
These observations are subject to be modified by the discovery of new material 
but are certainly of very great interest in their present shape. The diverse modes of 
extreme reduction in the different families are summarized as follows : The Tricono- 
dcmtidcB^ lose the lateral ( 1 ) incisor and first premolar and the reduction in the molar 
series is from behind forwards. The AmphitheriidcB lose the fourth ( 1 ) incisor, the 
second pmmolar is suppressed and the reduction in the molars is from behind forward. 
The suffer atrophy of the foremost three premolars. The Slylaeodonti- 
* Excepting the Phascolotheriinse. 
