'rKopiDor.Ki'i'is Fauna m' Canaxdaicta I.aki., X. \' . lL'7 



Nepionic Shell. — la the nepionic stage the shell is transversely oval 

 to siibcircular, with a hinge width less than the width below. Both 

 valves are convex and smooth. The hinge line is somewhat arcuate. 

 In the early part of this stage the shell is distinctly wider than long^ 

 but just before the inception of the plications the length and breadth 

 are about equal. Some of the dimensions are: .14 x .20 mm., .57- 

 X .64 mm., .66 X .70 nnn., .74 X .74 mm. 



Changes During Development. 



In Outline. — In the earliest neanic stages the shell becomes longer 

 than wide and this form is maintained until the shell reaches a length 

 of from four to seven millimeters, after which the proportions are re- 

 versed, and the width exceeds the length in succeeding stages. In the 

 adult condition the shells are usually 

 about one fourth wider than long. The 

 index followed through all stages from 

 the nepionic to the gerontic, shows a 

 double curve. Thus: Nepionic, 1.42, 

 1.12, I ; neanic, .87, .81, .79, .85, 

 .go, I, 1.12 ; adult, 1.12, i.iS, 1.25 ; 

 gerontic, 1.33. 



From the earl}' neanic through the 

 adult stages the width of the shell at 

 the hinge equals or exceeds the great- 

 est width below. In these stages the 

 cardinal extremities are usually mucronate. In gerontic stages the 

 width continues to increase without a corresponding growth on the 

 posterior margin, which produces rounded cardinal extremities and 

 gives the shell a transversely elliptical shape. 



Convexity of Valves. — Up to a length of from .75 mm. to i.oo 

 mm. both valves are slightly convex, the pedicle valve somewhat the 

 deeper. At this point, where the plications generally begin the 

 brachial valve becomes slightly concave, and in all later stages, is 

 more or less concave beyond the growth line marking the end of the 

 nepionic shell. In adult shells the dorsal valve is sometimes strongly 

 concave, but is usually only moderately so, or nearly fiat. From the 

 earliest plicated stage the median plication of the ventral valve is 

 somewhat stronger than the others and there is a corresi)onding sinus 

 in the brachial valve. As the older stages are reached, this median 



Fr;. 30. Tropidoleptus cariiialjis 

 Conrad. Series showing cliange in 

 relative convexity of valves. X 9- 



