16 
MUSEUM BULLETIN NO. 21. 
the variable features which have been observed, relate chiefly 
to the contour of the shell, character of the strise, and the spondy- 
lium in the ventral valve. 
The ventral valve shows three well marked types of contour, 
viz. (1) Strongly convex with a more or less clearly defined 
median ridge extending from the umbonal region to the front of 
the shell. (2) Very convex with tumid umbonal region round- 
ing regularly from the median region to the lateral and anterior 
margins without trace of median ridge. (3) Strongly convex 
in median and anterior region with or without median ridge, but 
with a broad shallow sinus extending from the beak across the 
umbonal region. These three types of contour make striking 
contrasts when individuals in which they are best developed are 
compared; but the intermediate forms in which neither the 
presence of ridge or sinus nor their entire absence can be positively 
stated, make difficult any attempt to discriminate them as dis- 
tinct varieties. They might be conveniently designated as 
varieties a, b, and c of the species C. decussatum according as the 
fold is present or absent or the umbonal sinus is developed. 
The radiating costae are generally best developed in the 
median region of the ventral valve, becoming much weaker or 
entirely obsolescent along the posterior half of the lateral 
margin of the valve. Fine concentric striae cover the entire 
surface of the shell. In many specimens both in the dorsal and 
ventral valve, radiating striae are obsolescent in the umbonal 
region except near the median line, the lateral striae appearing 
anterior to the umbonal region and being most strongly developed 
near the front of the shell. 
The collection contains a few small specimens which repre- 
sent early ephibic stages of this species. They lack the gibbosity 
which characterizes the ventral valves of all gerontic individuals. 
An interesting feature of these immature shells is the nearly 
complete development of the spondylium which they show. 
One of the characteristic features of the species is the short spon- 
dylium, the length of which shows a ratio to that of the ventral 
valve usually of 1 to 5 or 6. In the small immature shells the 
ratio is 1 to 2.S or 3. Thus it appears that the spondylium 
reaches approximate maturity at an early stage while the radiat- 
