DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 381 



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 sub-elliptical, 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 strise, 

 differing very little in any of the three species. 



In tracing the development of M. bicostata, var. jjetila^ 

 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, 

 var. simplex^ changes very little except to increase in width 

 and add a pair of plications at maturity. Individuals of 

 S. 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, more 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, l)ut 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 individuals, but they do not serve to 

 close the fissure, which remains open in all stages of growth. 



*S'. crispus, var. simplex, reaches a width of 8 mm., and 

 *S'. crispus often measures 22 mm. in width. Occasionally a 

 specimen of S. crispus of the usual size is found with but 

 seven plications on the dorsal valve, suggesting a very large 

 example of the variety, or that the characters of the smaller 

 and simple form are sometimes continued far beyond the 

 period when they usually disappear. Also, the features both 

 of the species and variety may be combined in a single speci- 



