390 PALAEONTOLOGY OP NEW-YORK. 



The bases of the crura are shown at a, aud those projecting a short distance for- 

 ward make a somewhat abrupt retral curve, turning back in the direction indicated 

 by the dotted lines b ; and thence descending into the cavity of the dorsal valve, 

 follow essentially the curvature of its outline, as indicated at c, to a point anterior 

 to the middle of the length of the spiral curve. Here the branches of the loop are 

 given off at d; and these projecting vertically into the cavity, are turned abruptly 

 forward, and beyond the dotted line, form a solid pointed plate which projects far 

 towards the anterior limits of the spires at e. The posterior portions are produced 

 backwards, and gradually ascending, continue parallel and in close proximity as far 

 &sf, where they diverge, sending off a fillet on each side Avhich at g assumes 

 the curvature of the normal lamellaj of the spire, following the same course 

 into the cavity of the valve, and extending forward to the origin of the 

 process forming the loop, the}- are united to the proper spiral lamclltc at h, which 

 then continue simple as shown in their extension to i. 



I had originally united this form of Athyris with A. spiriferoides of the Hamilton 

 group, regarding it only as a variety due to physical and geographical influences; 

 but when the internal structure presents such wide differences, I can scarcely 

 reconcile it with specific identity, if the distinction of species is to be preserved 

 upon any ground except that of arbitrary authority. In the examples before us, if 

 ■we admit such external and internal variations as incident to the species, it seems 

 to me very difficult to say where these variations may end, and characters of true 

 specific importance begin. At the present time I am not prepared to regard this 

 wide departure in the features of important internal organs as one of variety merely; 

 and while the external differences alone might not have been a sufficient reason 

 for specific distinction, I must regard them when thus combined, as entitled to this 

 consideration. Unfortunately the condition of the specimens is such as to afford 

 only rare opportunities of studying the interior. 



The figures given (PI. 46, figs. 1-4) illustrate the general form and character 

 of the species as it occurs at the Falls of the Ohio. A farther comparison with 

 specimens from Iowa, which I have described as A. vt'Uafa, reveals -no external 

 differences beyond the smaller size of the last named species. Under those circum- 

 stances I prefer to refer the present one to the species already described, than to 

 add another name to the list of species. The specimens under examination present 

 many external characters in common Avith some of the European varieties of A. 

 concentrica. 



Geological formations and localities. The original specimens upon which A. 

 vittata was founded were from Iowa city, in rocks of the age of the Hamilton 

 group, and it occurs likewise at New-Buffalo in the same geological horizon. The 

 species of the form figured on Plate 46 occurs in the Corniferous limestone at the 

 Falls of the Ohio, at Charleston-landing and other places. 



