356 STUDIES IN EVOLUTION 



tioii less than one-half the height of the area, which truncates 

 the beak more strongly than in younger shells, and the del- 

 tidial plates show a defined thickened area below the per- 

 foration, often extending to the dorsal beak (Plate XVIII, 

 figure 22). 



Variations. — As already stated, the elongate and broadly 

 flabellate shells appear to be, in this species, neither common 

 nor genetic variations. Among the extraordinary develop- 

 ments are specimens with duplicate plications in the sinus, 

 and one showing but seven plications on the shell. Another 

 individual has the initial shell strongly defined by a varix of 

 growth, and shows on this portion ten plications, but in the 

 subsequent growth only eight plications are continued, these 

 alternating at the varix with those of the embryonic shell. 



Atrypa reticularis Linnaeus, 1767. 



(Plate XX, figures 12-20.) 



Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. Hist., 



p. 162, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 304, 1882. 



The abundance of this well-known species at Waldron has 

 afforded the means of studying its developmental stages with 

 very satisfactory results. All the individuals, from the earli- 

 est observed stage upward, agree in contour, there being no 

 such variation in this respect as has been noticed in some 

 other species (e. g., Hommosjnrci evax, Whitfieldella nitidd) in 

 which appear deviations from the normal, producing a long 

 type and a broad type. The youngest individual detected 

 has a length of 2.25 mm. and a width of 2 mm., though this 

 may not be regarded as the initial shell on account of the 

 presence of partially developed deltidial plates. From this 

 stage of growth to maturity, the material has afforded every 

 variation in size and structure. 



Specific Characters. 



Mature Form (Plate XX, figures 13, 13 a, 20, 20 a). — 



Atrypa reticularis is so widely distributed, historically and 



