Occasional Papers of the Museum of Zoology 25 



27-30, ^2, and ^^), while those in the right hand cokinin and 

 the middle figure in the upper row (figs. 31, 35-38) are evi- 

 dently of the male type. However, young shells are more or 

 less inseparable and in old age the proportions again often 

 become similar, as, for instance, the middle figure in the bot- 

 tom row (fig. 34) looks like a female, but the earlier growth- 

 lines give more the contour of a male. 



The older specimens are practically black and all are dis- 

 colored, but an application of oxalic acid to some of the 

 younger ones reveals a beautiful, silky-brown epidermis, often 

 with quite evident, olive-green rays. The beaks of all are 

 eroded, but, in two or three of the more nearly perfect speci- 

 mens, remains were observed that apparently indicate the beak 

 sculpture to consist of low, rounded wrinkles, with a slight 

 tendency to be doubly looped. The silky appearance is caused 

 by the close and regular arrangement of the growth-lines, 

 which are crossed at right angles (in many specimens) by fine, 

 radiating striations. At the posterior end, the latter become 

 coarser and more distinct and are often separated by quite 

 pronounced ridgelets. This structure appears to be a charac- 

 teristic of the surface of the shell-substance, and may or may 

 not affect the epidermis. In some of the older shells, the epi- 

 dermis has a similar, flaky appearance to that characteristic 

 of the group Artonaias. As shown by the figures and meas- 

 urements, the shell is extremely variable and the older, arcurate 

 specimens bear little resemblance to the younger shells. They 

 are all connected by intermediates, but the specimens figured 

 are chosen for divergence rather than resemblance. Fig. 32 

 shows an especially aberrant form, with much higher beaks, 

 which are very swollen. The right pseudocardinals are usu- 

 ally quite equal and compressed (as remarked by F. and C), 

 but often the upper is smaller (as pointed out by Simpson, 



