IO4 STUDIES IN GEOLOGY, No. 3 



the posterior keel, at which point it recurves sharply into the 

 elliptical anterior margin. Umbones prominent, inflated, 

 their apices twisted anteriorly, situated at the anterior third 

 of the full length. The upper surface of the beaks slopes 

 distinctly forward, and is indented by a sulcus which is 

 distinct on the inner side of the beaks, but which is lost on 

 the umbonal slope. The umbonal slope is rounded near the 

 beaks, the surface slightly excavated posteriorly, the keel 

 tending to be angular, and the anterior surface gently 

 rounded but full. Ribs 33 on each valve, equal in width to 

 the interspaces on the central part of the shell and much 

 wider posteriorly; square in cross-section, those of left valve 

 prettily noded, the nodes becoming indistinct to absent on the 

 posterior part of the shell; noding much less distinct, often 

 absent, on right valve except anteriorly. Near the anterior 

 ventral margin the ribs of the left valve are longitudinally 

 sulcate ; those of the right valve are only slightly sulcate ven- 

 trally on the anterior slope. The area is narrow, of medium 

 length, with two to three fine wave-shaped furrows and the 

 margin elevated behind the beaks. The teeth are in a very 

 straight, even row, and are distally oblique. Dimensions of 

 2 specimens: Length, 24.5; height, 18; (semi) diam., 8.75; 

 length of hinge, 17 mm. Length, 25.5 ; height, 19.25 ; (semi) 

 diam., 10; length of hinge, 18 mm. 



This form is very closely related to Area proletariat 01 and 

 A. intumulata Pilsbry & John., 102 from the Oligocene of 

 Santo Domingo. Comparison with the types of these forms 

 brought out the following differences, which are here given 

 in critical detail on account of the extreme similarity in gen- 

 eral appearance of the forms: in A. proletaria the ventral 

 margin descends posteriorly, being nowhere parallel to the 

 hinge, whereas in singewaidi it is in general almost parallel ; 

 the posterior margin of proletaria meets the hinge at an 



lM Proc. Ac. Nat. Sc. Phila., vol. 69, p. 188, 1917. 

 102 Idem., p. 187. 



