598 SYSTEMATIC PALEONTOLOGY 



to be found at Devizes and Blackdown, with the upper valve. It is pos- 

 sible that these are different species from those in the Chalk, the costae are 

 less prominent, and the stria? more distinct; at present, however, I can 

 consider them only as varieties. Tab. 56, fig. 3, represents a specimen in 

 ferruginous sandstone from Chute, which may possibly prove to be a dis- 

 tinct species. Its length exceeds its breadth by one-fifth, and on the sides 

 of the larger costae are two lesser ones, which are partly blended with them ; 

 the surface is nearly smooth. I have only seen this specimen." Sowerby, 

 1812. 



Shell rather large for a Cretaceous Pecten; cordate, very strongly inequi- 

 valve, subequilateral, lower valve highly convex, the upper flattened or 

 feebly concave ; maximum diameter at or a little behind the median hori- 

 zontal; umbone of right valve very prominent, evenly inflated, rising well 

 above the hinge line, orthogyrate ; dorsal margins diverging at an angle 

 of approximately 90, produced so that the ventral and lateral margins 

 subscribe an arc of only about 180 ; external surface of lower valve sculp- 

 tured with five or rarely six elevated, evenly rounded primaries, subequal 

 in size and spacing and between each pair three or four more or less equal 

 secondaries; submargins sculptured with rather fine close-set radials fi.ve 

 in number, as a rule ; ornamentation of upper valve more uniform in char- 

 acter, usually of twenty to twenty-five subequal and equispaced, well 

 rounded and elevated radials ; incremental sculpture fine and sharp ; hinge 

 line rather short, not far from five-ninths of the maximum latitude, over- 

 hung by the umbo of the right valve ; auricles only slightly unequal, the 

 anterior a little more produced and relatively lower and less strongly 

 lirate than the posterior ; posterior auricle receding below the hinge line, 

 the anterior feebly constricted to form the byssal notch ; characters of 

 interior of shell not known. 



The identity of the American species with the European has been ques- 

 tioned since the day of d'Orbigny. The Maryland representation is very 

 meager and offers very little assistance toward the solution of the problem. 

 As in Pycnodonte vesicularis the true affinities of the group should be 

 worked out once for all by an exhaustive study of material from all the 

 representative localities. If the two forms prove distinct Sowerby's name 



