48 MEMOIRS OF THE STATE MUSEUM 



In the several series selected from the material at hand, it is evident 

 that the shell assumed the characters and form of maturity when reaching 

 a length of about 10 mm. At this period of growth, the fold of the dorsal 

 valve becomes elevated, and the sinus of the ventral valve depressed 

 (fig. 15, plate 4). Previous to this stage, the dorsal valve is depressed 

 and transversely concave, and the plications of the opposite valve are raised 

 along the median line of the shell. 



SPECIFIC CHARACTERS 



Mature Form (plate 4, figs. 14, 14^, 14^, 22, 220). Shell triangular, 

 cuneiform, widest across the pallial region. Length equal to about twice 

 the depth of the valves. Beaks compressed laterally, attenuate and pointed. 



Ventral valve moderately convex, subangular along the latera, marked 

 by a deep sinus, which commences near the middle of the length, and 

 becomes very marked in front, depressing three plications, of which the 

 middle one is detruded more than the others. 



Dorsal valve convex, gibbous in the posterior part, with the latera 

 elevated and subangular ; marked in front by a prominent fold which begins 

 near the beak as a depression carrying four plications, of which the two 

 central ones are usually much more elevated than the other pair. 



Area high, closed by two triangular deltidial plates. Perforation of 

 the ventral beak ovate, truncating the apex, and limited below by the del- 

 tidial plates. 



Surface marked by from eight to ten strong, angular plications, which 

 are crossed by very fine, regular, sharp, concentric striae. Mature specimens 

 usually measure from 10 to 17 mm. in length. 



Incipient Form (plate 4, figs. 12, 120). The youngest shell detected 

 has a length of 1.5 mm., is flattened, and nearly circular in outline. The 

 dorsal valve is depressed in the middle, and carries four plications. The 

 beak of the ventral valve is broadly triangular, exsert, and elevated, with a 

 triangular, open area without deltidial plates. 



