252 Annals of the Carnegie Museum. 



ous specimens from various localities on Valcour Island, Valcour, Platts- 

 burgh, Chazy, and Crown Point show this stage of development, and 

 this might be styled the normal form of the species. Specimens of 

 this sort are from 6.5 to 9.5 mm. in length (Plate XXXVI, fig. 21). 

 Other specimens of about the same size, from Sloop Island, show 

 an adult condition in which there are three plications on the fold, 

 and two in the sinus, but there is also a plication on each side of the 

 fold and sinus (Plate XXXVI, figs. 25, 26). The adults which have 

 passed through all stages of development are found in abundance on 

 Sloop Island. Specimens are from 10 to 14 mm. in length, have three 

 plications in the sinus, four on the fold, and one or two pairs on each 

 valve outside the fold and sinus (Plate XXXVI, figs. 33-36). 



Billings mentions specimens from the Mingan Islands with five 

 plications on the fold and four in the sinus, but such specimens- have 

 not yet been found in the Champlain Valley. 



A variety which can not be correlated exactly with any of the 

 above stages occurs on Sloop Island. The best specimen is 10.2 mm. 

 long and 7.3 mm. wide, which is rather a narrow form for this species. 

 The sinus of the pedicle valve contains one plication, and the dorsal 

 fold two, as is usual in stage 2, but in addition there is a plication on 

 each side of the fold and sinus. In this case the formation of plica- 

 tions in the fold and sinus was arrested at an early stage, but the 

 development at the sides continued. This is of especial interest in 

 connection with Camarella vnlborthi of the Black River, which has 

 three plications on the fold, two in the sinus, and two or three pairs 

 of plications at the sides. This species may have been derived from a 

 variety of Camarella varians in which the development of plications 

 in the fold and sinus was arrested at stage 3, while the development 

 of plications at the sides went on beyond any stage noted in C. varians. 



Description. 



Shell subtriangular in outline, the greatest width toward the front. 

 Both valves convex, the ventral beak extended a little beyond that 

 of the opposite valve; both beaks incurved. The sinus in the pedicle 

 valve has from one to three plications, and the fold from two to four. 

 Most individuals do not have any plications outside the fold and sinus, 

 but a few have one or two pairs on each valve. The majority of the 

 specimens have three plications on the fold and two in the sinus; a 



