LOPHIODONTlDiE. 625 



respectively. Superior incisors with flat, transverse crowns. Superior first 

 premolar without inner lobe. All the other molars with two external 

 tubercles in the form of compressed cones. The true molars and last pre- 

 molar have an additional external tubercle in front of the anterior cone, 

 which is an elevation of the basal cingulum. It is variously developed on 

 the second and third premolars. Second premolar with the inner lobe like 

 a wide cingulum ; the third and fourth premolars have one internal tubercle, 

 and one or two smaller ones between it and the external tubercles, the 

 anterior of which forms, on wearing, with the inner tubercle, a single trans- 

 verse band of dentine. True superior molars with two internal and two 

 intermediate tubercles, which form on wearing, oblique transverse bands. 

 The anterior one of these passes anteriorly to the base of the anterior 

 external cusp. 



The first and second inferior pi-emolars have no internal tubercles. 

 The third and fourth have them on the anterior pai-t of the crown, but not 

 on the posterior. The true molars have two pairs of opposite tubercles, 

 the third and last adding a posterior fifth lobe or heel. The internal 

 tubercles of the true molars have a circular section ; the external ones are 

 V-shaped in section. The posterior limb of the V is directed toward the 

 internal posterior tubercle; the anterior limb extends diagonally toward 

 the other internal tubercle. The limbs of the anterior V have correspond- 

 ing directions, the anterior terminating at the anterior border of the crown 

 of the tooth. 



The species of Hyracotherium difi"er in the relative developments of the 

 intermediate tubercles of the superior molars, of the distinctness of the 

 tubercles of the inferior molars, of the lengths of the diastemata, and of 

 the development of the cingula. 



In this genus the nasal bones are well developed, and join the pre- 

 maxillaries to form the boundaries of the external nares. There is a strong 

 postorbital process of the os-frontis, which does not inclose the orbit jjoste- 

 riorly by reaching the zygomatic arch. The latter jjresents no postorbital 

 process upward. The postglenoid process is rather short and obtuse. The 

 meatus auditorius externus has no osseous floor external to the petrous 

 ■chamber. The paroccipital process is well developed, and is distinct from 

 40 c 



