REPORT OF THE STATE PALEONTOLOGIST I9O3 287 



the beaks are at the angle, and are therefore well forward ; the hinge 

 line carries the middle portion of the dorsal line above the flat plane 

 of the umbones and gives the shell a very slightly alate appearance ; 

 posterior margin about twice the length of the anterior and quite 

 regularly curved, the sharpest bend being found at the posteroven- 

 tral margin; ventral margin very slightly convex, a little more 

 straightened in middle portion and forming an angle of about 25° 

 with the general dorsal area as viewed across the shell; the basal 

 line forms an angle with the margin of the anterior truncate part 

 of a little less than 90° and the curve of the basal margin gradually, 

 and regularly increases till it meets the margin of the anterior trun- 

 cate portion in a well rounded angle. Extreme breadth of shell 3mm 

 and at a point but little anterior to the middle and very closely half- 

 way between dorsal and ventral margins. Beaks incurved and nearly 

 touching, byssal pit just below them and the cause, in part, of the 

 truncate appearance. Surface very regularly curved, the usual 

 oblique ridge from beaks to posterior margin not prominently 

 marked. Concentric growth lines very fine and numerous but not 

 easily seen. 



Genus cyrtodonta Billings 



Cyrtodonta? lam^Uosa sp. nov. 



Plate 4, figures 10-13 



Description. Shell of moderate size, its length being 20.7mm; 

 length of hinge line lomm ; a perpendicular from posterior extremity 

 of hinge line reaches posteroventral angle and measures i6mm. 

 (posterior hight of shell) ; posterior margin convex and quite 

 closely forming the arc of a circle of lomm radius with center on 

 axis of greatest length ; the arc extended forward would follow the 

 shell for about one fourth of dorsal margin and then enter the shell 

 again at or very close to the beaks ; ventral margin but little con- 

 vex, nearly straight to point directly below beaks, anterior hight 

 7.7mm; anterior margin at first following the gentle curve of the 

 ventral margin, but becoming markedly convex when it rounds back 

 toward the beaks ; the outline of the shell with the exception of the 

 segment cut off by the straight hinge line and the projection of the 

 anterior margin closely resembles the outline of the gibbous moon. 



