138 Annals <jf thk Carnegie Mlseu.m. 



The plications are introduced in pairs on the laterial margins, each 

 pair coming in outside the older ones. The plications are added on 

 both valves at practically the same time, so that not counting as 

 plications the two ridges which bound the sinus in the ventral valve, 

 nor the fold on the dorsal, there are always the same number on both 

 valves. On a specimen 2.43 mm. long and 3.40 mm. wide, there 

 are two plications on each valve. On one 3.13 mm. long there are 

 6 ; on one 3.33 mm. long there are 8 : at 4.76 mm. long there are 

 10 • at 7 mm., 12; and at 9.13 mm., 16. 



In adults there is a depressed line along the middle of the fold, 

 giving it a flattened and, sometimes, divided appearance. 



Pedicle Opening. — In the nepionic stage the pedicle opening is 

 large and oval and is shared by both valves. In early neanic stages 

 it becomes restricted to the ventral valve, in which there is a narrow 

 triangular delthyrium bounded by dental lamellfe which unite with a 

 median septum a short distance in front of the area. This spondy- 

 lium is visible at the beak of a specimen less than i mm. in length 

 and continues to grow throughout life. In adult specimens it reaches 

 down about two thirds of the distance from the apex to the cardinal 

 line. The median septum projects behind the point where the dental 

 lamellae are joined to it and from its posterior edge two thin partitions 

 run diagonally forward and join the dental lamellae, thus forming a 

 V-shaped ridge in the bottom of the spondylium. There are two 

 triangular cavities between these partitions, the dental lamellae and 

 the median septum, but they do not appear to have an opening on 

 the external surface of the shell at any stage (Fig. 45). 



In the neanic stage at which there are two plications on each valve, 

 /. c. , when the shell is from 1.6 to 2.4 mm. long, narrow deltidial 

 plates are formed, which, at a later stage, when the shell is from 3.0 

 to 3.5 mm. in length, coalesce along the median line and continue 

 to cover the lower part of the delthyrium throughout life. A pedicle 

 opening is left at the apex. 



Comparison ivitJi Specimens from OtJier Localities. 



The mature specimens from Canandaigua Lake are from 6 to 9 mm. 

 in length and 8 to 12 mm. in width, and bear from 8 to 12 plications. 

 The hinge width is generally less than the greatest w^idth, and the 

 ventral area is inclined either backward or forward. P'or comparison 

 wath this material, si^ecimens from York, Livingston Co., and Eighteen 



