BRACHYTHYRIS 375 



Description. Shell above medium size, suborbicular in outline, a little 

 wider than long, the hinge-line shorter than the greatest width of the shell, 

 the cardinal extremities rounded or obtusely angular. The dimensions 

 of a nearly complete, but small, undistorted specimen are : length 40 

 mm., width 46 mm., thickness 27 mm., length of hinge-line 32 mm. The 

 dimensions of a large pedicle valve are: length approximately 56 mm., 

 width 62 mm.., convexity 23.5 mm., length of hinge-line 45 mm., height of 

 cardinal area 6 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, the umbonal region rather narrow, the surface usually somewhat 

 compressed towards the cardinal extremities; beak pointed and rather 

 strongly incurved; cardinal area rather low, arched, the curvature be- 

 coming greater towards the beak, the lateral margins defined, sloping 

 from the beak to the cardinal extremities with a slightly sigmoidal curve, 

 the delthyrium rather large, wider than high, bordered on each side 

 by a thickened ridge which is raised above the general level of the area ; 

 surface of the area marked by longitudinal lines of growth ; lateral slopes 

 of the valve each marked with 10 to 12 depressed, more or less obscure, 

 simple plications, those near the sinus being broad, the successive ones 

 towards the cardinal extremities becoming narrower and fainter, the 

 last four or so becoming so faint, especially in large individuals, as to be 

 almost obsolete; mesial sinus defined to the beak, anteriorly it becomes 

 less sharply defined, is rather broad, of moderate depth, and rounded 

 in the bottom, at the anterior margin it is sometimes produced into a 

 nasute extension, it is marked in the median line by a narrow, ill-defined 

 obscure or almost obsolete plication, and by one or two on each side 

 which are broader but equally obscure. 



Brachial valve less convex than the pedicle, its greatest convexity near 

 the middle, the surface compressed towards the cardinal extremities, the 

 beak projecting slightly beyond the hinge-line ; mesial fold defined to the 

 beak, where it is narrow and scarcely elevated, becoming broader and 

 often rather strongly elevated towards the front, rounded in contour, 

 marked by four or six very obscure, sometimes, apparently, almost or 

 quite obsolete plications; lateral slopes marked by plications which are 

 similar in number to those of the opposite valve, but which are commonly 

 much more strongly developed. 



The minute surface markings of the shell have not been clearly seen, 

 but the shell seems to be smooth except for the presence of more or less 

 strongly developed lines of growth. 



Internally the muscular impressions of the pedicle valve are but 

 faintly developed, and the dental plates are so short that they are rep- 

 resented in the internal casts as but slightly depressed grooves, one on 

 each side of the delthyrium. 



