74 MEMOIRS OF THE STATE MUSEUM 



SPECIFIC CHARACTERS 



Mature Form (plate 7, figs. 3, 3^). Shell comparatively large, ven- 

 tricose, broadly ovate or subpentagonal. 



Ventral valve gibbous in the umbonal region, with a low, broad dorsum 

 extending from the umbo to near the middle of the valve, where it becomes 

 flattened, sinuate, and at the anterior margin is -reflected dorsally into a 

 linguiform extension. Beak closely incurved over the dorsal valve, fully 

 concealing the foramen. Cardinal slopes angulate, and slightly excavate. 



Dorsal valve evenly convex, somewhat gibbous, strongly arcuate trans- 

 versely along the dorsum, which becomes elevated into a low fold, deeply 

 emarginate in front for the reception of the extension from the opposite 

 valve. Beak obtuse, incurved and concealed. 



Surface smooth, marked by concentric growth-lines near the margin. 



Occasionally, individuals of large growth show a greater length than 

 breadth, presenting an elongate form, but this variation seems to be due to 

 more rapid axial growth after the attainment of adult size, and does not 

 manifest itself in the incompletely developed shells. 



Incipient Form (plate 7, figs, i, i, 2, 20). The minute shell which 

 appears to have been the initial form for the species, has a circular outline 

 and depressed convex valves. The ventral valve is evenly convex, with 

 the beak erect, short, and broad. The cardinal area is low, the foraminal 

 aperture triangular, reaching to, but not encroaching upon the beak. The 

 deltidial plates are absent. Dorsal valve with the beak not incurved but 

 inconspicuous. Neither valve bears any trace of a median elevation or 

 depression. 



DEVELOPMENTAL VARIATIONS 



General F'orm and Outline. An inclination toward a lenticular form 

 and circular outline is noticeable in all immature individuals. Until a size 

 of about 1 8xi 8 mm. is attained, there is rarely, if ever, any trace of the 

 strong marginal fold of maturity. 



