TETRACAMEEA 217 



and meeting at the beak in an acute angle, the anterior and antero-lateral 

 margins subsemicircular, the line of junction between the two valves 

 deeply and sharply serrate, especially along the anterior and antero-lat- 

 eral margins. The dimensions of an internal east of a pedicle valve are : 

 length 22.9 mm., greatest width 20 mm., convexity 4.8 mm. The dimen- 

 sions of an internal cast of a brachial valve are : length 22.7 mm., width 

 19.4 mm., convexity 8 mm. 



Pedicle valve nearly flat throughout the greater portion of its surface, 

 gently convex in the umbonal region, abruptly deflected along its postero- 

 lateral margins and only a little less abruptly deflected along the anterior 

 and antero-lateral margins; mesial sinus essentially obsolete or repre- 

 sented only by a slight sinuosity of the margin upon the anterior deflected 

 portion of the valve; beak acutely pointed, only moderately incurved; 

 plications twenty to twenty-six in number, rounded or subangular, usually 

 simple and extending from the beak to the margin, but occasionally with 

 one or two intercalated plications anterior to the beak, the plications be- 

 coming successively smaller towards the postero lateral margins, those 

 upon the deflected postero-lateral surfaces of the valve curving rather 

 strongly towards the margin. Internally the dental plates are well de- 

 veloped and are joined near the inner surface of the valve to form a spon- 

 dylium which is supported by a well defined median septum which reaches 

 anteriorly about one-third the length of the valve, from the outer surface 

 of each dental lamellae a lateral buttress plate passes to the inner surface 

 of the postero-lateral deflected portion of the valve, the buttress plates 

 extending anteriorly from the beak for about one-third the length of the 

 median septum. 



Brachial valve similar in contour to the pedicle, with the postero-lateral 

 surfaces abruptly deflected to meet the similarly deflected portions of the 

 opposite valve, and the anterior and antero-lateral margins also deflected 

 in a manner similar to that of the opposite valve, the non-deflected sur- 

 face of the valve somewhat more convex than that of the pedicle valve, 

 and the depth of the valve a little greater ; mesial fold obsolete except for 

 the slight sinuosity in the anterior margin ; the beak acutely angular and 

 incurved beneath that of the opposite valve; plications similar in form 

 and number to those of the pedicle valve. Internally the median septum 

 extends anteriorly from the beak for from one-third to nearly one-half 

 the length of the valve, the lateral lamellae which support the inner mar- 

 gins of the divided hinge-plate join the inner surface of the valve at the 

 base of the median septum so that the three lamellae make but a single 

 median slit in the internal casts of the valve. 



The minute surface markings not preserved upon the internal casts, to 

 which observations were limited. 



