14 



i. External cingulum robust. 

 /? Anterior median crest little or not distinct. 

 Larger ; median crests cut off externally ; no anterior marginal tubercle ; 



external faces impressed A. brachylophum. 



Small ; posterior median crest confluent with external crests ; an anterior 

 marginal tubercle ; external face little impressed A. longicristis. 



j3j3 Anterior median crest isolated. 

 Larger ; a large anterior marginal tubercle ; posterior marginal linear ', 



wrinkled A. condoni. 



Small ; anterior marginal tubercle minute, posterior triangular ; median 



crests short ; smooth A. cuneatum. 



ii. External cingulum narrow. 

 External faces without median rib ; median crests short, the anterior cut 

 off ; marginal tubercles small A. exoletum. 



Stylonus seveksus, gen. et sp. nov. 



Gen. Char. These are derived from superior molar teeth Stylonus is 

 allied to Hlppotherium 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 

 extinct equine animals. It carries to its limit the line of development 

 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 

 AneMppus, 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 AneMppus, Hippotlierium, and Stylonus, is a side branch 

 from that which terminated in Equus. The line of Eqwus must be traced 

 from Ancldtherium through Protohippus and Hippidium. 



Specif. 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 each side of the 

 latter are uniformly concave, and contain a rather shallow deposit of 

 cementum. 



