CRAN^NA 251 



Surface of both valves marked only by concentric lines of growth which 

 vary in the strength of their development. Shell structure finely and 

 closely punctate. 



Remarks. This is a small species and may be easily recognized by its 

 more or less rotund form and by the absence of fold or sinus. Specimens 

 of it have sometimes been identified as Dielasma formosa, but besides being 

 entirely different from Dielasma' in the essential characters of the interior 

 of the rostral portion of the brachial valve, it is a comparatively broader 

 shell with the valves more regularly arched transversely. The species 

 shows considerable variation in outline, especially in the position of the 

 line of greatest width, it being sometimes anterior and sometimes poste- 

 rior to the mid-length ; it also varies in the convexity of the valves, in 

 some old individuals becoming ventricose. The species never attains a 

 great size, about 17 mm. being the maximum length which has been 

 observed. 



Horizon. Burlington limestone. 



CRAN^NA SULCATA n. sp. 

 Plate XXXIV, Figs. 55-59 



1895. Dielasma turgida Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 81, 

 figs. 1-3 (not figs. 4-8). 



Description. Shell small, ovate, subpentagonal in outline, the anterior 

 margin emarginate, longer than wide, the greatest width near the mid- 

 length of the shell. The dimensions of a nearly perfect individual are: 

 length of pedicle valve 11 mm., length of brachial valve 9.2 mm., greatest 

 width 8.8 mm., thickness 8.1 mm. 



Pedicle valve strongly convex, the greatest convexity near or posterior 

 to the middle, the surface almost regularly arched from the beak to the 

 front or with the curvature a little more convex posteriorly, the surface 

 curving abruptly to the lateral margins becoming inflected to the cardinal 

 extremities ; mesial sinus originating near the middle of the valve, rather 

 narrow or of moderate width, rounded in the bottom, sometimes becoming 

 deep at the anterior margin, its lateral margins rounding into the lateral 

 slopes of the valve without sharp demarcation ; the beak pointed, rather 

 strongly incurved, perforated by a subcircular foramen which encroaches 

 wholly upon the umbonal region, coming in contact with the delthyrium 

 only at its apex ; delthyrium broadly triangular, closed by deltidial plates 

 throughout, except as it is filled with the beak of the opposite valve. In- 

 ternally the dental lamellae are well developed and of moderate length. 



Brachial valve a little less convex than the pedicle, the greatest con- 

 vexity near the middle, the surface arched from beak to front along the 

 median line with the curvature sometimes more abrupt posteriorly and 

 sometimes anteriorly, the curvature to the lateral margins rather abrupt ; 



