CRAN^ENA 



249 



plate into three subequal parts or into two equal lateral divisions and a 

 broader central one, and in some species the crural ridges are accom- 

 panied by similar ridge-like thickenings upon the opposite side of the 

 hinge-plate. The brachidium is short and Dielasma-like, not reaching to 

 the middle of the valve. 



FIG. 24. A series of ten cross-sections of the rostral portion of the shell of 

 Crancena iowensis (X 2^), only one of which (A) shows the pedicle valve 

 with the dental lamellae. In the brachial valve is shown the concave hinge- 

 plate, perforated posteriorly, with the crural ridges passing anteriorly into 

 the crura. 



Remarks. This genus differs fundamentally from Dielasma in the 

 origin of the crura from the thickened crural ridges of the hinge-plate, 

 rather than from the crural lamellae resting upon the inner surface of the 

 valve, and in the absence of a special plate in the bra.cb.ial valve for mus- 

 cular attachment, the muscles being attached directly to the inner surface 

 of the valve. From Girtyella the genus may be distinguished by the ab- 

 sence of a median septum in the brachial valve and by the origin of the 

 crura from crural ridges on the hinge-plate rather than from the anterior 

 extension of the inner extremities of the socket plates. 



CRANCENA GLOBOSA n. sp. 

 Plate XXXIV, Figs. 60-66 



1895. Dielasma Burlingtonensis Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 



pi. 81, figs. 9-11. 

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



figs. 14-15 (not figs. 12-13, 16-26). 



Description. Shell small, subovate in outline, longer than wide, the 

 greatest width near the mid-length, the valves subequally convex. The 

 dimensions of two complete examples are : length of pedicle valve 14 mm. 

 and 10.5 mm., length of brachial valve 12 mm. and 9.1 mm., greatest width 

 10.3 mm. and 8 mm., thickness 9.4 mm. and 5.8 mm. 



