Tropidolki'its l'\\rNA Ai' Caxandakua I,akf., N. v. 117 



side the median septum are low and do not end in prominent pro- 

 jections. 



In C. scitiiltis the ridges are very prominent and usually pustulose. 

 The muscle scars are not strongly marked, but the reniform part of the 

 brachial impression is deeply excavated and is limited b}' concentric 

 rows of strong pustules. In front of these rows of pustules all around 

 the margin of the inner surface of the shell there is a border which is. 

 almost smooth. 



Spines. — The spines on this species are better preserved than on 

 any of the others in this material. They are larger and coarser, and 

 are also ])rotected by lying against the shell and each other. 'I'he 

 smallest shells in the collection show the bases of two pairs of spines, 

 and the adults show remains of from four to seven pairs. The average 

 distances at which they emerge are as follows: ist pair, .53 mm. ; 

 2d, r.i6 mm. ; 3d, 2.01 mm. ; 4th, 254 mm. ; 5th, 3.41 mm. ; 6th, 

 4.40 mm. ; 7th, 5.53 mm. 



Gerontic Stage. — With old age the shell becomes greatly thickened, 

 the muscle scars more deeply impressed, there are numerous strong 

 varices of growth near the margin, and the stria; become faint or en- 

 tirely obliterated in the front. The shell becomes gibbous, the width 

 below becomes greater than that at the hinge, and there is a constric- 

 tion just in front of the cardinal area which gives a sort of auriculate 

 appearance to the cardinal extremities. When this occurs the plane 

 of the ventral area is revolved inward from its normal position. 



Comparison itnth Speeiniens from Oi/ier Localities. 



The specimens from Pompey Hill, N. Y., have about the same size 

 and index as the above, but usually have fewer plications. A speci- 

 men 7.5 X IO-5 nim. has 18 plications, and one 7 X 9.5 mm. has 22, 

 while individuals of that size from Canandaigua Lake have from 25 to 

 30. The largest specimen is 10x14 mm. and the smallest 4X5 

 mm. The index in adults is from 1.22 to 1.40 mm. 



Three specimens from Pratt's Falls, N. Y., are of fair size, 8x11 

 mm, to 9 X 12 mm., and have from 22 to 30 stripe. The spines ex- 

 tend nearly parallel to the hinge. 



In the collection there are numerous specimens from Eighteen Mile 

 Creek, most of which are smaller than the average specimens from 

 Canandaigua Lake. The largest one is 8 x 13 mm. with 35 strice, and 

 the smallest is 5.5 x 8 mm. An average specimen is 7.5 X 10 mm., 



