Tropidolkpius Fauna ap Canani)AI(;ua Lakk, N. Y. 103 



Char\!^es During Development. 



Outline. — There is very little change in the outline except as it is 

 affected by the irregular growth. In the nepionic stage the shells are 

 circular but become wider than long in the earliest neanic stage and 

 remain so through all succeeding stages. The width at the hinge is 

 always less than the width below. The index is from 1.25 to 1.50 

 mm. in neanic stages and 1.50 to 1.80 mm. in adult and gerontic 

 stages. 



Convexity of Valves. — The dorsal valves of young shells are nearly 

 flat or slightly convex, but become more convex in the adult when 

 the greatest convexity is about the middle of the valve. The ventral 

 valve is always more convex than the opposite one and is often rather 

 sharply deflected in front. 



Fig. 21. Ort/iothetes c/u'i>iitngei!sis andsfrintiis \\a\\. iJiagiam of a dorsal valve 

 to show the 12 primary striK. Between two of them the secondary, tertiary and 

 quaternary stria; are filled in, to give an idea of the uniform apjjearance produced by 

 the crowding of the stris. X i- 



Fig. 22. O. chemitngensii pectenacea Hall. Dorsal valve, showing earlier plica- 

 tions. X 12. 



Stricc. — The shell of the variety pectenacea has, up to a length of 

 I to 1.2 mm., 13 to 15 simple, sharp striae separated by spaces 

 which are wider than the strine. Then there appear four or six new 

 strii^i in the middle of the front, implanted, one in each interspace 

 (Fig. 22). Later more are added until there is one between each pair 

 of the original stride. The next step is the appearance of strife, again 

 in pairs, one implanted on each side of each of the secondary strire. 

 At a still later stage more pairs are added, one on each side of the 

 next previous ones to appear. So in the adult we have, considering a 

 single unit of striae, first, two striae extending from the anterior margin 

 to the smooth portion of the shell at the beak ; second, a single stria- 

 tion bisecting the area between the first two but not extending to the 

 beak ; third, a pair, one on each side of the bisecting striations but 

 not more than half as long ; and fourth, four more stri^, one in each 

 of the spaces between the preceding ones. These last ones are usually 



