94 Annals of the Carnegie Museum. 



The diductors are bounded behind, and at the sides by two low, 

 pustulose ridges, and the whole interior of the valves outside the scars, 

 is covered with smaller pustules. 



In the dorsal valve there are two pairs of adductor scars bounded 

 posteriorly and laterally by ridges, but not well limited in front. 

 The anteriors are situated between the posteriors and are divided from 

 them by faint ridges. 



There is, as far as could be seen, very little change in these scars 

 from the young through the adult stages. In the dorsal valve, the 

 ridges, which limit the posterior and lateral sides of the scars, are 

 more divergent in young stages than in the adult, thus producing 

 relatively narrower scars in the adult than in the neanic specimens. 



Unfortunately no specimens showing strong gerontic characters 

 have been obtained from this material. 



Pholidostrophia iowaensis Owen. 



Hall, Pal. X. Y., IV, 1867, p. 104, pi. 18, lig. I. 



In the collection there are about 130 specimens, ranging in size 

 from 1.6 X 2 mm. to 14 x 16 mm. Three specimens retain both 

 valves in their proper relations, but the remainder are separate valves. 

 The specimens figured by Hall are adult and gerontic individuals. 

 The smallest is 11.4 x 14 mm. and the largest 13 x 17 mm. 



Dcsciipiion of Smallest Shell. — The smallest shell in the collection 

 is 1.6x2 mm., semi-elliptical in form, with a width at the hinge 

 greater than the width below. The ventral valve is gently and regu- 

 larly convex, while the dorsal valve is convex at the beak and concave, 

 nearly flat in front. The surface is smooth on both valves. 



Changes During Development. 



Outline. — During the neanic stages the cardinal extremities are 

 alate and the hinge width is usually one and one-half to twice the 

 length of the shell. In adults the width at the hinge equals or is a 

 little less than the width below, and the length and width frequently 

 become very nearly equal. The young shells are not, however, so 

 strongly alate as the young of Stroplieodonta perplana, but this is largely 

 due to the accidental removal of the tips of the cardinal extremities. 

 One specimen, a ventral valve, shows three stages of growth, and the 

 spiniform extremities of each are fairly vvell preserved. Instead of the 

 new growth being added to the whole length of the hinge, these ex- 



