38 MEMOIRS OF THE STATE MUSEUM 



ventral valve, thirteen, and for the dorsal, fourteen plications. Dimensions 

 of an average example, length, width and depth, 9, 8 and 5 mm. 



A single individual presents the only important abnormality noticed, 

 viz.: a failure to produce the requisite plications upon the latera, the dorsal 

 valve bearing but five, and the ventral, six. Of these, three are on the 

 fold, two in the sinus. It is interesting to notice that in the umbonal 

 region the normal number of plications had been formed in their 

 regular arrangement ; their disappearance on the latera and irregular 

 disposition in fold and sinus took place abruptly upon the completion of a 

 growth-line 2 mm. from the apex. This is a marked instance of reversion 

 after the assumption of certain adult features. 



Incipient Form (plate 4, figs. 6, 6). The example with which 



our series opens measures -75X.5 mm. It is elongate subtriangular, with 

 the ventral beak elevated and erect, the cardinal margins sloping for two- 

 thirds the length of the shell ; foramen triangular, slightly, if at all, en- 

 croaching upon the apex, without deltidial plates, margins thin ; dorsal 

 beak rounded, inconspicuous. At one-third the distance from the apex to 

 the anterior margin, fine thread-like plications appear, four upon the dorsal, 

 and three (five ?) upon the ventral valve. The median sulcus on the 

 dorsal valve is broader and deeper than any other, forming the embryonal 

 sinus, and is accompanied by a correlatively strong plication on the oppo- 

 site shell. 



DEVELOPMENTAL VARIATIONS 



General Form and Outline. The form of the shell varies from 

 dimensions in which the length is one-quarter greater than the width, to 

 those of maturity when the width is slightly greater than the length. The 

 depressed, subspatulate embryo eventually becomes convex and deep. 

 The embryonal sinus and fold on the dorsal and ventral valves respectively, 

 are never so prominent as in R. indianensis, and soon become lost, the 



