112 AXNALS OF IHK CaRNECIIK MuSEU.M. 



The individuals in the material from East Bethany, N. Y., also 

 average larger than the Canandaigua Lake specimens. The largest 

 specimen is 21.5 x 30 mm. and has 125 stri?e. The smallest is 10 x 15 

 mm. with 50 stride and five pairs of spines. A specimen of about 

 the same size from the silicified material has 80 strire. These speci- 

 mens do not show the sinus in the ventral valve. 



The Eighteen Mile Creek, N. Y., specimens are more clearly the 

 size of the individuals from Canandaigua Lake. The smallest is 

 11X15 mm. with 54 stride. A larger one is 15.5 x 23 mm., with 

 eight i)air of spines and 82 striae. The largest specimen is 18 x 25 

 mm. Some of these shells show a sinus in the ventral valve and a 

 fold in the dorsal valve. Others do not have either. 



Chonetes scitulus Mall. (Plate IV.) 



Hall, Pal. N. v., IV, 1867, p. 130, pi. 21, tig. 4. 



Of this species there are about 3,500 specimens, among which are 

 individuals showing all stages of development. The smallest shell is 

 .93 mm. long and i.oi mm. wide. The largest 6 mm. long and 9 

 mm. wide. 



The smallest specimen figured by Hall, {57X9 mm. and the 

 largest (from the Hamilton) is 8.4 x 12 mm. The Chemung speci- 

 mens are, according to that author, usually larger. A figure of the 

 protegulum of this species has been published by Beecher {Atn. Jour. 

 Sci., Vol. XLI, 1891, p. 357, pi. 17, fig. 14). It is oval in outline, 

 slightly longer than wide, and has an arcuate hinge. 



Nepionic Shell. — The nepionic shell is subcircular in outline with 

 a hinge width a little less than the greatest width below. In the 

 ventral valve there is a narrow, shallow sinus, bounded by two ridges. 

 In the dorsal valve there is a median fold which extends to the beak 

 and, on either side, a lesser fold which does not reach the beak nor 

 join the median fold. The lateral folds are less distinct than the 

 median one and die out toward the front. 



The length of the shell at this stage varies from .42 to .56 mm. 

 On a specimen froni Thedford, Ont., the ventral shell at this stage is 

 .42 mm. long and .46 mm. wide. 



Changes During Developinenf. 



Outline. — Until the shell reaches a length of about r.5 mm. the 

 length and width are nearly equal, and the inde.x is from 1.08 to 1.20. 



