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PALAEONTOLOGY OP NEW-YORK. 
The bases of the crura are shown at a, and these 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 
as f where they diverge, sending off a fillet on each side which at g assumes 
the curvature of the normal lamellae 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, they are united to the proper spiral lamellae at h, which 
then continue simple as shown in their extension to i. 
I had originally united this form of Athyris with A. sjpiriferoides 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. vittata , 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 with some of the European varieties of A. 
cpncentrica. 
Geological formations and localities . The original specimens upon which A. 
vittata was founded were from Iowa city, in rocks of the age of the Plamilton 
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. 
