66 MEMOIRS OF THE STATE MUSEUM 



intercalary addition of a single plication on one side of the median plica- 

 tion of the dorsal valve. 



A tendency to obesity is often manifested by the shell, at or before 

 reaching the average dimensions of maturity, when it may be supposed 

 that the full growth of the individual has been attained. This obesity is 

 produced by a rapid thickening of the shell at the margins, making the 

 anterior face truncate and forcing the ventral beak over the dorsal in the 

 same manner as if the valves were forced to open along the hinge. It is, 

 therefore, only in individuals which have reached this obese condition that 

 the ventral beak is incurved. 



DEVELOPMENTAL CHANGES. 



The character of the primal or elemental shell may be seen from a 

 single example (plate 5, fig. 18), in which the plications are abruptly 

 developed at a distance of 1.5 mm. from the apex, and, presumably, that 

 portion of the shell within this limit, represents approximately the size of 

 the original embryonic shell. This portion of the individual is quite smooth, 

 and shows but a trace of the median fold and sinus. As already observed, 

 there is a marked permanency in the surface features of the species from 

 early youth to maturity. The smallest individual obtainable bears, as in 

 the mature condition, six plications on the ventral and five on the dorsal 

 valve, though those near the hinge-line are quite faint. 



The beak is prominent and exsert, except in obese shells, where it is 

 incurved. In the earliest stage where the character of the foramen is well 

 preserved the individual has a length of 4 mm. and a width of 3.5 mm. 

 Here it is seen to be elongate-oval, the deltidial plates having formed to 

 such a degree as to be in contact with each other and to have anchylosed, 

 so that the median suture is detected with difficulty. The lateral sutures 

 always remain distinct even to maturity, and it is evident that the union of 

 the plates with the shell along these joints has not been as firm as in many 

 species, as it is not infrequently found that the plates have been displaced 



