136 PROCEEDINGS OF THE NATIONAL MUSEUM vol.106 



Horizon and locality: In sandy clay of Zone 5 above oyster shell 

 layer, 4 feet above beach level, South Chesapeake Beach, Calvert 

 Count}^, Md. Calvert formation, upper Miocene. 



MANDIBLES 



Although the dimensions of the preserved portions of these mandi- 

 bles are slightly smaller than those of USNM 13471, their conforma- 

 tion as well as the size of the alveolae resemble rather closely the 

 mandibles of the latter. These mandibles obviously belonged to an i 

 immature individual since the symphysis has not as yet ankylosed I 

 and the condyles are narrower. Approximately 43 mm. of the 

 symphyseal region of the left mandible is preserved. 



Two teeth were preserved in situ in the left mandible and one in 

 the right mandible (pi. 13). In the left mandible, 11 alveolae (in an 

 interval of 110 mm.) were located behind the level of the posterior end I 

 of the symphysis, and in the right mandible 12 alveolae (in an interval i 

 of 120 mm.) were so situated. The largest alveolae at the rim vary in i 

 anteroposterior diameter from 7 to 8 mm. and in transverse diameter 

 from 6 to 8 mm. The roots of the teeth were implanted nearly verti- 

 cally in the alveolae. The maximum interval between the alveolae ' 

 is approximately 3.0 mm., but for most of them it is not more ' 

 than 2 mm. 



The distance from the posterior end of the symphysis to the hind- 

 most alveola (110 mm.) on the left ramus is slightly less than the ' 

 corresponding interval on USNM 13471, and the distance from the 

 posterior end of the symphysis to the posterior face of the condyle ' 

 (283 mm.) is also less than that of USNM 13471 (306 mm.). 



On the outer surface of the right mandible (pi. 14, fig. 1) nutrient 

 foramina, from which grooves of varying lengths extend forward, , 

 were located below the 6th, 8th, 12th, and 13th alveolae (counting ' 

 forward from the hindmost). Nutrient foramina on the outer 

 surface of the left mandible (pi. 14, fig. 2) are located below the 

 6th, 9th, and 12th alveolae. 



The slope of the dorsal margin of each ramus from the coronoid : 

 process to the hindmost alveola corresponds closely with that of 

 USNM 13471, although the dorsal border of this process is less 

 noticeably thickened. The distance from the apex of the coronoid 

 process of the left mandible to the hindmost alveola is 139 mm. 



The large orifice for the dental canal on the inner surface of the 

 mandible was located at least 35 mm. behind the level of the hindmost 

 alveola. The ventral borders of both mandibles are incomplete in 

 the region of the angle. Both condyles are unusually narrow, the 

 transverse diameter being 16 mm., and the dorsoventral axis (33 

 mm.) of each condyle is directed cMiauely outward. 



