46 EOCENE MOLLUSCA. 
“(a) Coste 24, 25, round, finely imbricated with intermediate costelle, imbrications 
continued over the interstices. Interstices deep; shell sub- 
orbicular. 
(4) Costas 24—28, round, imbricated; imbrications more distant than in a; 
interstices shallow; costellz occasional; shell tranversely sub- 
orbicular. 
(c) Costa 18—32, round, finely imbricated; costellz occasional; shell nearly 
orbicular. 
(d) Costz acute. 
(e) Multistriatus ? 
(7) Tripartitus ? 
(yg) ‘Transversely sub-orbicular, coste 30, coarsely imbricated. 
(2) Coste round, smooth.” (dwards.) 
My own observations pretty nearly coincide with the above, and my formula will 
stand thus: 
a. Ribs 25-—82, rounded and smooth, without an intermediate ray. The smooth- 
ness may possibly be from attrition or decortication. 
6. Ribs 24—82, rounded and imbricated, without intermediate ray. 
6 4 25—28 with 5 o 
9 2? 
d. ,, 24-28 tripartite, imbricated all over, without intermediate ray. 
29 
@. 4, 24—28 a with bs $ 
8 acute, subcarinate, and slightly imbricate, without intermediate 
ray. 
Although we have both attempted the above divisions, I fear the lines of separation 
will be anything but distinct between these variations. 
One perfect specimen, with the two valves united, shows the ribs quite smooth and 
rounded; another, equally perfect, has the younger portion of the shell with smooth ribs, 
while the outer and older portion is strongly imbricated both over the ribs and between them ; 
and on another specimen the younger portion is imbricated, and the older portion smooth, 
uniting thus in one individual the characters of what are called varieties. Where the 
intermediate ray is developed, it is generally most prominent at the margins. I believe the 
figures of Mr. Sowerby, above referred to, are representations only of varieties of this 
variable species. The 40-radiatus I imagine to be one of the lesser number of ribs, with 
the intermediate ray elevated into a primary one. The diameter of the shell is generally 
a trifle in excess, measuring from the pedilateral to the siphonilateral margin ; but in some 
specimens the proportions are decidedly reversed. The auricles are rather large and 
