ATHYRIS OF THE HAMILTON GROUP. 
287 
From this loop on the ventral side there is a bifurcation, from which 
proceed accessary lamellae which are intercalated between the first and 
second turns of the spire, as shown in the illustration under the generic 
description. 
The figures on Plate 46 present the principal varieties in form known to me in 
the Hamilton group. The young individuals in this formation are usually pretty 
symmetrical, as shown in figure 5. They are generally not strongly lamellose, but 
sometimes acquire this character before attaining their full size, as shown in figures 
6, 7 and 10. 
In figure 8 we have an extremely oblate form, where the width is much greater 
than the length; and figure 9 shows a subquadrate regular form. We have similar 
extremes in figures 11 and 12, where the first is a little longer than wide; and the 
second, having the same length, has a much greater width and less depth. These 
figures illustrate the extremes of form among the young and half grown indivi¬ 
duals; while the full grown specimens of the oblate form are shown in figures 
13 and 14. 
Figures 15 -19 illustrate the larger or apparently full grown forms of mode¬ 
rate gibbosity ; while figures 20-24 are of the more gibbous forms. Since these 
■varieties of form occur in the same localities, both being quite common, it is pos¬ 
sible that they may indicate sexual differences. 
Figure 25 is an extremely large form of moderate convexity and closely lamel¬ 
lose surface. 
The casts present equally great extremes in form and proportions; and the 
muscular and other interior markings are subject to slight variation. 
The dimensions of this shell vary, from specimens much smaller, than figure 5 
and having a length of a quarter of an inch, to those having a length of about 20 
lines with a width of 22 lines, while the depth is less than 10 lines, or about the 
same as that of figures 22 and 23. 
After an examination of many hundreds of specimens, showing.all the extremes 
of form and character, I can find no means of specific distinction among them. 
I have heretofore expressed an opinion that this species is distinct from the 
Athyris concentrica of Europe, judging from the more straight hinge-line and less 
prominent beak of the ventral valve, as well as that the greatest breadth is above 
the middle of the shell. In the European species also, according to figures and 
specimens, the ventral valve is always proportionally a little more convex than in 
the American specimens. I have scarcely seen any illustration of the interior of 
European specimens; but the cast of a dorsal valve given by Mr. Davidson in his 
Monograph of British Devonian Brachiopoda shows a comparatively narrower 
and more elongate muscular area, a form not approached in any degree by the 
