34 1 Dominican Fossils — Maury 177 



oms, discrepant on the two valves; anterior margin rounded, ex- 

 tending beyond the end of the hinge; ventral margin rounded; 

 posterior straight. Shell externally very like A. cor-cupidonis 

 but very much thinner, and that species has a sinuous posterior 

 margin, rather broadly rounded at the end of the umbonal ridge 

 while this shell has a straight posterior margin; is less rounded 

 at the base, and the posterior end of the hinge line is longer and 

 extends upward, giving the shell a winged appearance although 

 it is not alate as that term is used in the true Arks. In A. cor 

 cupidonis the cardinal area is inclined upward from the top of the 

 hinge to the beaks making them appear high; in this species the 

 cardinal area is flat posteriorly and anteriorly slopes downward 

 from the hinge so that as seen from the interior the point of the 

 beak is about level with the upper margin of the hinge. The 

 beaks are also less twisted; exteriorly the beaks of both appear 

 high and narrow; the cardinal area in both has two or three 

 grooves and elevated margins; but the hinge of A. riocanensis is 

 longer, more markedly angled at the ends, and much narrower, 

 the teeth proportionately short and delicate. Length 25, altitude 

 23, semidiameter 1 1 mm. This shell may attain a larger size- 

 Though superficially similar to A. cor-cupido?iis it is really a very 

 different species. 



Locality. — (Exp'd '16) Zone H, Rio Cana at Caimito. 

 (Not common). 



Scapharca Pennelli Gabb 

 Plate 29, Figure 8 



Judging from the specimens in our Gabb collection, Gabb 

 had two distinct forms confused in his species A. Pennelli. It 

 seems best to limit that designation to the shell which fits his 

 description. As shown by the figure which is of one of Gabb's 

 specimens, it is characteristically more oblique, thinner, and the 

 ribs less beaded than the other form which we have named >S. 

 Arthurpennelli. A. Pennelli apparently is comparatively a rare 

 shell, the other species being the one w r hich we found so abund- 

 ant. 



