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 j'oung 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 maj' indicate sexual differences. 



Figure 2o 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 

 mu.scular 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 

 Athyriii 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. Davi1)SON 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 



