ioo STUDIES IN GEOLOGY, No. 3 



ing to meet the hinge at an angle of 135. Umbones narrow, 

 erect, very slightly prosogyrate, inflated, 6 mm. distant. 

 Umbonal slope broadly rounded anteriorly, more acutely 

 rounded mesially, a gentle keel being followed by a slightly 

 excavated posterior surface on the left valve ; the right valve 

 is not excavated posteriorly. Valves meet marginally in an 

 acute angle ventrally and posteriorly. Ribs, 24 in number 

 on each valve, square, concentrically noded on left valve 

 with cog-like corrugations, the posterior ribs being nearly 

 smooth; on the right valve the ribs are not noded except 

 anteriorly, but are crossed by numerous fine lines of growth. 

 The ribs are separated by interspaces about equal in width 

 mesially and posteriorly, but much wider than the ribs 

 anteriorly. Area lozenge-shaped, a little longer behind the 

 beaks than before, plain except for one lozenge-shaped fur- 

 row two-thirds of the distance from the beak to the hinge, 

 crossed by numerous very fine longitudinal lines and bounded 

 by a marginal furrow whose outer wall is slightly elevated. 

 Length, 30.5; height, (left valve) 28.5, (right valve) 30; 

 diameter 27; length of hinge, 17.5 mm. 



This species is clearly transitional between the true Scap- 

 harcas and the Cunearcas. It has the shape and discrepant 

 valves of Cunearca (although the right valve is the 

 larger instead of the left), and the area is entirely smooth 

 save for one furrow. It probably represents a stage in the 

 development of the divergent stock at which the chief 

 characteristics of the Cunearcas have assumed definite shape, 

 with the trace of areal furrowing as a disappearing vestig- 

 ial inheritance of Scapharcan characteristics. It is most 

 closely allied to A. (Scapharca) cor-cupidonis Maury, 93 from 

 the Miocene (?) of Santo Domingo, agreeing with it entirely 

 in all characters save the following: the area is equally wide 

 before and behind the beaks, not discrepant, has but one 



93 Bull. Amer. Pal., vol. 5, No. 29, p. 175, pi. 30, figs. 5, 6, 7, 

 1917- 



