Tropidoleitl's Fauna at Canandaicua Laki:, N. Y. 109 



Nepionic Shell. — In the nepionic stage the shell is almost circular 

 in form, the hinge line slightly curved and almost as long as the 

 greatest width below. Both valves are convex, the 

 ventral more so than the dorsal. In the ventral 

 valve there is a narrow, well defined sinus, and a 

 corresponding fold occurs in the dorsal shell. The 

 length of the nepionic shell varies from .30 to .45 Fig~24. Chonetes 

 mm. and the width is slightly greater (Fig. 24). lor onat us {Com&A). 



Dorsal valve of a 

 Ckani^es Diirin}:^ Development. young specimen, 



Outline. —The outline of shells less than 2 mm. ^li""'i"g protegu- 



. . . , , ^ 1 , , \vi\w, nepionic shell 



in length is quite variable. In some, the length .,, , ,r,j , 



t" 1 ' o with dorsal fold, and 



nearly equals the width, while others are from one- the shell of the pau- 



third to one-half wider than long. In general, the ciplicate neonic 



shell becomes wider, in proportion to its length, as "^tage. The radial 



it grows older. The index varies from i . 09 to i . 49. ' ' " ^ ' represented 



Shells less than 6 mm. wide usually have an index of .. 



■' ■ tions. X 9- 



less than 1.30. In the larger shells the index aver- 

 ages about 1.40. From the beginning of the neanic stages the hinge 

 width equals or exceeds the width below. In adult and senile speci- 

 mens the greatest width is below the hinge. 



Convexity of Valves. — The ventral vahe is moderately and e\enly 

 convex throughout all the stages. The donsal valve is convex during 

 the nepionic stage, but becomes concave immediately in front of the 

 smooth part of the shell and continues slightly concave to the front. 



Strict. — The smallest plicated shell seen has eight strong strire on 

 the dorsal valve and nine on the ventral. The shell at this stage, is 

 .65 mm. long and i.oomm. wide. The first strice on the dorsal valve 

 appear to be formed by the bifurcation of the fold of the nepionic 

 shell, and on the ventral valve the first four are formed by the bifur- 

 cation of the two ridges which bound the sinus in that valve. Then 

 four more strias are added on each valve, two on each lateral margin 

 and two strice are implanted on the dorsal valve, just outside the two 

 strioe formed by the bifurcation of the dorsal fold. A single striation 

 is implanted in the sinus of the ventral valve as the ventral shell con- 

 forms to the depression in the dorsal shell, caused by the bifurcation 

 of the fold. From this point, striae are added on the lateral margins 

 until there are twelve or fourteen on the dorsal valve and thirteen or 

 fifteen on the other. At this stage, the shell is usually about 3 to 



