Occasional Papers of the Museum of Zoology 25 
27-30, 32, and 33), while those in the right hand column 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, 
