24 TEETIAET VEETEBEATA OF THE FAYtTM. 



(see text-fig. 6, p. 18) ; at the same time the difference is less than that between the 

 upper premolars and molars. They are four in number, the three posterior ones 

 being double-rooted. Pm, 1 is not known in A. zitteli, but in A. andrewsi its 

 crown is very like the others ; in both species it has a single root. 



Each premolar consists of two columns separated externally by a deep vertical 

 groove. In early stages of wear the pattern is a double V, but later the concavity of 

 the V's becomes obliterated, and the inner face of the teeth is nearly straight. In 

 some unworn specimens the postero-internal end of the anterior V is seen to 

 project somewhat across the opening of the posterior V so as partially to close it : 

 this projection seems to correspond with the metastylid of some other ungulate teeth 

 (e. g. Hyrax \_Procavia\, Eguus). The cingulum is well developed on the inner 

 face of the teeth, and is also present in some specimens, though not all, on the 

 outer side between the main columns. 



The closely crowded canines and incisors are, so far as known, closely similar 

 to one another. They are high columnar teeth, convex outwards from above 

 downwards, and nearly " rectangular in section. The outer face of the crown is 

 strongly convex from side to side ; on the inner side the cingulum is well developed, 

 and between it and the inner face of the tooth there is a deep, more or less 

 developed pit or " fosette." 



The lower milk-dentition is imperfectly known, and in such specimens as are 

 preserved it appears that the milk-molars are practically identical in form with 

 the molars of the permanent series. 



Vertebral Column. — In correlation with the great size and weight of the skull the 

 atlas (text-fig. 8) is very large and massively constructed, its width being especially 

 great. The condylar cups {cond.) are large and widely separated both above and 

 below ; they are strongly concave from above downwards, and are much wider above 

 than below, where they narrow rapidly as they pass on to the ventral bar. The lateral 

 surfaces for the axis {asc.) consist of a subcircular, slightly concave upper portion, from 

 the outer and lower side of which a less well-defined triangular area runs down 

 to the ventral bar, nearly reaching the outer border of the odontoid face [od.). 

 This is very broad, sharply defined, slightly concave from side to side, and prolonged 

 somewhat backwards on a median prominence of the ventral bar, the posterior 

 angle of which forms a blunt hypapophysis. 



The neural arch is wide both from side to side and from before backwards ; 

 its upper surface is strongly convex from before backwards and forms a slight 

 prominence on either side of the middle line {ns.). Laterally, close to its anterior 

 border, the arch is perforated by the narrow slit-like passage which transmits the 

 first spinal nerve {sp.f.). This foramen opens into a groove, the posterior border 

 of which forms the anterior edge of the large transverse process [t.p.). This is very 



