SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 1 3 



apex directed forward, and the posterior face is abraded by the lower 

 lateral incisor. 



A considerable diastema separates the incisor from the next succeeding 

 tooth, p-. The premolars increase in size posteriorly, an increase which 

 is sudden between p- and p-, and gradual behind that. The first pre- 

 molar is quite small and implanted by two roots ; the crown consists of a 

 single external lobe, which has a cordate and slightly convex outer face, 

 and a very low internal enamel ridge, or cingulum, which is divided by a 

 median sulcus into anterior and posterior portions. The second premolar 

 is very much larger and is inserted by three roots ; the crown is formed 

 essentially as in p-, but the external face has two convexities separated by 

 a shallow median depression ; the internal ridge is better developed and, 

 in addition, there is an internal cingulum. The third premolar is still 

 larger and almost molariform, but has only three roots, of which the internal 

 one is very large ; the external wall consists of two equal, concave cusps, 

 which unite to form a median ridge, but has no distinct anterior or pos- 

 terior style ; the internal ridge is distinctly, but incompletely divided into 

 three cusps, the deutero- and tetartocones and the anterior conule, the 

 posterior conule being a minute pillar attached to the inner side of the 

 postero-external cusp (tritocone) ; the cingulum varies in prominence and 

 completeness. The fourth premolar is like p-, but larger, and the posterior 

 conule is much better developed and the extremely large internal root 

 shows signs of an incipient division into two. 



The molar series is shorter than the premolar. The first and second 

 molars have a similar pattern and differ only in size, m- being the largest 

 of the series. The crown consists of two equal external lobes, the para- 

 and metacones, with concave outer faces, which meet in a prominent meso- 

 style, but, though the anterior and posterior borders are raised, there is no 

 distinct para- or metastyle. The two internal cusps, proto- and hypo- 

 cones, of which the former is somewhat the larger, are more or less con- 

 ical, or pyramidal in shape : the anterior conule is connected with the 

 antero-internal cusp (protocone), forming with it a very oblique transverse 

 crest, but is demarcated from the antero-external cusp (paracone) by a 

 deep, narrow, vertical cleft, while the posterior conule is a slender, iso- 

 lated pillar, connected at the base with the postero-external cusp (meta- 

 cone). From the postero-internal cusp (hypocone) runs a narrow promi- 

 nent ridge, perhaps an elevated cingulum, to the hinder edge of the 



