MARTINIA 421 



the cardinal extremities rounded, the greatest width near the mid-length 

 of the shell. The dimensions of three nearly perfect specimens are : 

 length of pedicle valve 28.3 mm., 21.5 mm. and 20.7 mm., length of brachial 

 valve 24 mm., 18 mm. and 17.6 mm., maximum width 24.5 mm., 20.9 mm. 

 and 20.9 mm., 'length of hinge-line 13 mm., 12.2 mm. and 9 mm., thickness 

 19.8 mm., 14 mm. and 14.3 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, the umbo prominent and projecting posteriorly beyond the hinge- 

 line, the surface curving abruptly to the cardinal margin, more gently to 

 the lateral margins and most gently to the anterior margin ; mesial sinus 

 originating in the umbonal region, shallow or of moderate depth, rounded 

 in the bottom or subangular along the median line, ill-defined laterally, a 

 little produced anteriorly ; beak rather prominent, strongly incurved ; 

 cardinal area small, strongly arched, the lateral margins not sharply de- 

 fined, the surface curving almost without interruption into the lateral 

 slopes of the valve except towards the cardinal extremities; delthyrium 

 large, broadly triangular, occupying one-half or nearly one-half of the 

 hinge-line at its base. 



Brachial valve less convex than the pedicle, the greatest convexity 

 posterior to the middle, the surface curving abruptly to the cardinal mat- 

 gin and sloping with a gentle convexity from the median line to the lat- 

 eral margins, convex along the median line from the beak to the front 

 with an increasing curvature posteriorly, sometimes a little compressed 

 towards the cardinal extremities; mesial fold ill-defined, scarcely differ- 

 entiated in the posterior half of the valve, it is rounded on top and often 

 has a slight mesial depression in the anterior half of the valve ; beak small, 

 incurved ; cardinal area very narrow. 



The surface markings of both valves, as shown upon exfoliated speci- 

 mens, consist of fine, radiating costae, about 2 or 3 of which occupy one 

 millimeter near the margin, becoming finer towards the beak, these costae 

 increase usually by implantation and are of such strength that they must 

 have been present upon the surface of non-exfoliated shells ; crossing the 

 radiating costaB are exceedingly fine concentric lines which are usually 

 crowded at intervals, especially in the anterior half the shell, in stronger 

 lines of growth. 



Remarks. In the original description of this species the presence of 

 radiating markings is noted but they are described as being too fine to be 

 observed with the unaided eye. Upon the specimens here described the 

 radiating costae are very distinct, agreeing more closely with the shells 

 from Ohio which Whitfield has referred to the species than to the orig- 

 inals of Meek and Worthen. The specimens observed exhibit consider- 

 able variation in their proportional length and width, -and in the angular- 

 ity of the mesial sinus, the specimens with the more angular sinus having 



