88 Annals of thk Carnkgie Museum. 



there is a narrow fold which extends about half way to the front. 

 Otherwise the shell is smooth. The width at the hinge is a little less 

 than the width l)elow. The length of an average specimen is .42 

 mm. and the width .54 mm. 



Chaiii^es During::; Development. 



Outline. — Immediately after the nepionic stage the width at the 

 hinge becomes greater than the width below and remains so through- 

 out all succeeding stages. The cardinal extremities are most alate 

 during the adolescent period and all immature forms are characterized, 

 when perfect, by very long hinge lines. In the senile state, the car- 

 dinal angles are rounded a little, but the width at the hinge is still 

 greater than the width below. From the early neanic through the 

 adult stages the index increases gradually, while in senile specimens 

 the anterior growth lowers it again. It varies from 1.25 to 1.50 in 

 adolescent individuals and from 1.50 to 1.75 in adults. In one senile 

 specimen it falls as low as 1.19. 



Convexity of Valves. — In the nepionic stage both valves are con- 

 vex, but when a length of about .50 mm. is reached, the dorsal valve 

 becomes concave in front and follows very closely the contour of the 

 other valve throughout the succeeding stages. Shells from i to 6 

 mm. long are very slightly convex, sometimes almost flat, but as they 

 grow older, the convexity increases, until, in the gerontic stage, they 

 are almost hemispherical. 



Stria. — The smallest specimen showing stri;^ is i.i mm. long and 

 1.8 mm. wide. It is a ventral valve and has nine strong striae, be- 

 tween which are extremely fine stride which can hardly 

 be made out. Five of the prominent stride extend 

 back to the line bounding the nepionic shell. One 

 of these is on the median line of the valve and has 

 P , ,. a pair on either side of it. Between each pair of 



pkeodonla iuceqiiis- these striiTC a striation is implanted near the anterior 

 triata (Conrad), margin of the shell (Fig. 5). 



Ventral valve, After this stage the number of stride increases rap- 

 showing the prim- j^jy^ ^^ implantation. An adult has from 30 to 60 

 ary striae. X 8- • • • 1 1 n r 1 ,- ,. 



prominent stnte with bundles of three to five smaller 

 strire between them. They are not obliterated in the senile stages, but 

 increase in number with the increase of the shell in size. There are, 

 in that stage, from 50 to 80 of the strong striae. 



