76 MEMOIRS OF THE STATE MUSEUM 



resemblance becomes more than superficial, for, in young shells of less than 

 2 mm. in length, it is difficult, and sometimes impossible, to refer them to 

 any one of the three groups. A general expression of the common charac- 

 ters is furnished by the young of S. crispus, van simplex, illustrated by 

 figure 4, on plate 6. 



Taking vS". bicostatiis, van pctilns as the simplest form, the young shell 

 is found to be nearly circular in outline, with a single, broad, median fold 

 on the dorsal valve. Passing to S. crispus, van simplex, of the same size, 

 the outline is seen to be broader, and there is an incipient plication on each 

 side of the median fold. The outline is still broader in .S". crispus, becoming 

 decidedly subelliptical, and the two lateral plications on the dorsal valve are 

 nearly equal in strength to the median fold. The surface ornamentation 

 consists of fine spinulose, or granulose, concentric striae, differing very little 

 in any of the three species. 



In tracing the development of S. bicostatus, van pctilns, the shell is found 

 to retain its embryonic characters up to full growth, neither materially 

 changing its form, nor adding to the primitive number of plications. 

 Likewise, .S'. crispus van simplex changes very little except to increase in 

 width and add a pair of plications at maturity. Individuals of 6". crispus 

 develop parallel to the variety simplex, up to a length of 5 mm., or until 

 about two-thirds the size of full-grown examples is attained. Subsequently, 

 ^rnore plications are added, increasing the number from three or five to 

 eleven, but otherwise, the general features of the shell are unchanged. 

 Even the relative convexity of the valves remains the same at all periods. 



In the incipient forms, the cardinal line extends for about one-fourth 

 the width of the shell, and at maturity measures three-fourths of this width. 

 The foramen does not develop at the same rate ; at first, it occupies one- 

 half or one-third of the ventral area, but advancing growth gradually 

 diminishes this ratio, until it is one-fourth or one-fifth the size of the hinge- 

 area. Two narrow, triangular, deltidial plates are present in full-grown 



