322 STUDIES IN EVOLUTION 



on its surface at maturity just about as many plications or 

 strise as its associate. 



There is an obese variation from the normal form of 

 R. liybrida, which was noticed by Professor Hall (loc. cit.), 

 and this appears early in the development of the species, 

 with a size of 3.5 mm. in length and 4.5 mm. in width, and 

 reaches a maximum growth with dimensions of 14 x 13 mm. 

 This variation is due to internal thickening and increase 

 in convexity, and is accompanied by abundant concentric 

 growth-lines which are as rare in the normal form as in 

 D. elegantula. A representative series of this species 

 affords variations between the following limits of size : .5 mm. 

 in length x .75 mm. in width (minimum), and 17 mm. in 

 length x 20 mm. in width (maximum). 



Leptcena rhomboidalis Wilckens, 1769. 



(PLATE XVI, figures 1-13.) 



StropTiomena rhomboidalis Hall. Twenty-eighth Ann. Kept. N. Y. State 



Mus. Nat. Hist., p. 151, pi. 22, figs. 4-10, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 288. pi. 22, 



figs. 4-10, 1882. 



This well-known species, although extremely abundant in 

 the mature state, is correspondingly rare in its undeveloped 

 condition. The young specimens which have been found are 

 nearly all more or less broken, and it is evident that while 

 young the shell was thin and delicate, consequently few of 

 their remains have been preserved. The series which has 

 been selected is, however, very complete in its representation 

 of the distinct phases of growth through which the individ- 

 uals pass in their development from youth to maturity. 

 The initial form, without radiating striae; the second 

 phase, a shell radiatingly striate, without undulations; the 

 third state, striated and concentrically undulated, but with- 

 out the angular geniculation of the valves in front; and the 

 last phase, with the full form and characters of maturity, 



