14 
i. External cingulum robust. 
f Anterior median crest little or not distinct. 
Larger; median crests cut off externally ; no anterior marginal tubercle ; 
external faces impressed............--. Suanekeee te A. brachylophum. 
Small; posterior median crest confluent with external crests ; an anterior 
marginal tubercle ; external face little impressed.......A. longicristds. 
23 Anterior median crest isolated. 
Larger; a large anterior marginal tubercle; posterior marginal linear 3 
wrinkled. .is ines an ddedowance cree ees Audet noaion ere ennine site A. condont. 
Small; anterior marginal tubercle minute, posterior triangular; median 
crests short; smooth............. euaG ey (ede euwewae es A. cuneatum. 
ii. External cingulum narrow. 
External faces without median rib; median crests short, the anterior cut 
off; marginal tubercles small. .............- wane A. exoletum. 
STYLONUS SEVERSUS, gen. et sp. nov. 
Gen. Char. These are derived from superior molar teeth  Stylonus is 
allied to Hippotherium in details, including the isolation of the anterior 
internal enamel covered column, which thus forms an island of dentine, 
and in the prismatic character of the tooth. It differs from it in the fact 
that the posterior internal column is isolated in the same manner as the an- 
terior, thus forming a second island on the triturating surface of the crown. 
This interesting new genus adds one to the already numerous forms of 
exvinct equine animals. It carries to its limit the line of dévelopment 
which retains the inner tubercles of the molar crown distinct from the 
median. The preceding station on this line which we know is the genus 
Anchippus, where the median crests have not assumed the antero-posterior 
direction belonging to the higher equine genera, and where the molars 
have short crowns and long roots. We may then believe that the line 
which includes Anchippus, Hippotherium, and Stylonus, is a side branch 
from that which terminated in Hquus. The line of Hguus must be traced 
from Anchitheriwm through Protohippus and Hippidium. 
Spectf. Char. Two superior molar teeth were accompanied by a num- 
ber of inferior molars as having been all found together, but whether they 
belong to one individual is uncertain. The dentinal lakes of the superior 
molar are confluent by the median transverse valley, and increased wear 
would probably join the posterior pair by their posterior angles. The bor- 
ders of the cementum lakes are simple, except one or two plications on 
their opposed adjacent borders, and one at the posterior inner part of the 
posterior. The internal columns are small, and their sections form two 
equal ovals with their long axes antero-posterior. The anterior dentinal 
lake sends off a narrow loop towards the posterior part of the anterior 
column. The shaft of the tooth is incurved, and the external face is un- 
equally divided by the usual ridge. The wide gutters on cach side of the 
latter are uniformly concave, and contain a rather shallow deposit of 
cementum. 
