48 
NAUTTLOIDEA. 
■ Trocholites planorbiformis, Conrad. 
1842. Trocholites piano rbifm'mis, Conrad, Journ. Acad. Nat. Sci. Phila- 
delphia, vol. viii. pt. ii. p. 274, pi. xvii. f. 1. 
1847. Trocholites planorbiformis^ Hall, Palaeontology of New York, 
vol. i. p. 310, pi. Ixxxiv. if. 3 a-f. 
Sp. Char. “ Depressed, orbicular, or planorbiform ; volutions 
about four or five, wider than deep [i. e. the transverse greater than 
the ventro-dorsal diameter] ; apex profoundly and equally depressed 
on both sides ; aperfure lunate ; section elliptical, with the inner 
side concave from the junction of the next volution ; surface marked 
by obliquely transverse ridges, which bend backwards, forming a 
broad curve [sinus] on the dorsal [convex] line [? side], longitudinally 
striated with rounded lines. 
In all the specimens examined, the outer lamina of the shell is 
exfoliated, and the fine striae of the surface are destroyed. The 
character of the shell is much like that of T. ammonius, and I have 
been disposed to regard it as a variety of the same. But in speci- 
mens of that species from the Trenton limestone I have rarely found 
the transverse and longitudinal ridges so strongly marked as in this 
specimen. The shell is always larger than the specimens of the 
Trenton limestone, but in other respects there are few important 
differences {Hall.) 
BemarTcs. The young in the Canadian examples of this species are 
ornamented with very distinct, but somewhat irregular, transverse 
annulations, which form a tolerably deep sinus upon the convex side 
of the shell. These ornaments become almost obsolete in the adult 
shell, though they sometimes reappear for a short distance ; they 
are accompanied by fine lines of growth. Crossing the latter, and 
running longitudinally for the whole length of the shell, are a series 
of very fine lines which form a minute network with the lines of 
growth. Two or three of the longitudinal lines, along the median 
line of the periphery, are stronger than the rest. 
Though there can be very little doubt as to the generic position 
of the fossil ascribed by Salter to the present species (Mem. Geol. 
Surv. 1866, vol. iii. p. 358, pi. xxv. f. 5), the fact that he describes 
the whorls as smooth, whereas in the American species they are 
ribbed, would be sufficient to distinguish the British species from 
the latter. 
The specimen figured by Blake (British Foss. Ceph. pt. i. pi. xxix. 
ff. 8, 8 a) shows a different style of ornamentation from that of the 
American examples above described, the ribs in the British specimen 
being much sharper, stronger, and wider apart than they are in the 
latter. Salterns type of Lituites Hibernicus is a cast only. 
