138 PROCEEDESrGS OF THE NATIONAL MUSEUM vol. los 



posterior diameter of crown at "base, 5.7 mm.; transverse diameter of 

 crown at base, 5.6 mm.; maximum diameter of root, 7.0 mm. 



The crown of the 10th tooth (counting forward from the hindmost) 

 in the right mandible has been broken off except for a portion of the 

 basal inner surface. The sculpturing this basal portion of the enamel 

 surface agrees with the teeth in the left mandible. 



Referred Specimen II 



USNM 20732: Two dorsal and one caudal vertebrae, one anterior 

 rib, eleven teeth, and left tympanic bulla. Collector, Norman H. 

 Boss, July 1920. 



Horizon and locality: Green marl, one mile south of Chesa- 

 peake Beach, Calvert County, Md. Calvert formation, upper 

 Miocene. 



TEETH 



Eleven teeth were associated with the three vertebrae and one rib. 

 Most of the teeth are essentially complete, although the crowns of 

 four are worn to varying degrees. These teeth, unlike those of 

 Tretosphys gabbii and Delphinodon dividum, have the enamel on the 

 crown very lightly ornamented, and the anterior teeth have notice- 

 ably elongated and nearly straight roots. 



To emphasize the differences existing between these teeth and those 

 of Tretosphys gabbii, the most ovious characteristics of the latter may 

 be summarized as follows : A number of the teeth possess a vertically 

 directed denticulated carina on the outer surface of the crown, while 

 others have the enamel on this surface ornamented with fine striae; 

 the posterior teeth, at least, have a fairly large accessory cusp with a 

 serrated cutting edge near the base of the posterior face; and the 

 inner surfaces of the crowns of many of the posterior teeth are more 

 noticeably rugose. In comparison to Delphinodon dividum, the teeth 

 of this porpoise possess gibbous roots with a more pronounced curva- 

 ture, and the posterior ones lack the strongly developed accessory 

 cusps on the inner face of the crown. 



On seven of the teeth the narrow basal border on the enamel of the 

 inner face of the crown is slightly elevated and the irregular upper 

 edge of this border appears to the eye to be indistinctly sculptured. 

 The crown curves inward and is not markedly compressed antero- 

 posteriorly. Two of the teeth have a pair of upwardly diverging 

 carinae that originate on one tooth (pi. 16, figs. 1, 8) in a series of 

 slightly elevated rugosities on the basal portion of the posterior face 

 of the crown, and on the other tooth (pi. 16, figs. 2, 6) near the middle 

 of the height of the posterior face of the crown. Both of these teeth 



