138 
NAUTILOIBEA . 
Helderberg Group, near Sandusky, Ohio, which strikingly resembles 
some of the Triassic species described by Mojsisovics (Med. Trias- 
prov.), and Nautilus Sangamonensis, Meek and ^\^orthen \ from the 
Upper Coal-Measures of Sangamon County, Illinois. 
CAUBONIFEUOUS SPECIES. 
Pleuronautilus falcatus, J. de C. Sowerhy, sp. 
1840. Nautilus falcatus, J. de C. Sowerhy, Trans. Geol. Soc. ser. ii. 
vol. Y. pt. iii.. Explanation of pi. xl. f. 9 ; in J. Prestwich, “ On 
the Geology of Coalhrookdale,” p. 413 of above volume. 
1854. Nautilus falcatus, Morris, Cat. British Foss. 2nd ed. p. 308. 
8]). Char. “Discoid, inner whorls exposed; the front convex; 
sides nearly flat ; the outer edge of each whorl obtusely angular, 
the inner edge inclined towards the preceding whorl, the space 
between crossed by many arched ribs ; septa simply concave : 
siphuncle central ; aperture nearly square, but G-sided, with two of 
the sides very small. Length of the aperture 1 inch. Diameter of 
the shell about 2| inches.” {Sowerhy.) 
Remarks. This species is remarkable for the quadrate form of the 
whorls and the inflexions of the sutures. These latter form a deep 
sinus on the sides of the shell and arching forward upon the lateral 
angles form an acute backwardly-directed lobe upon the periphery. 
A somewhat deeply-impressed zone upon the dorsal side of the shell, 
indicates the amount of overlapping of the whorls. The transverse 
cost[E correspond nearly in number, but not quite in their direction, 
with the sutures. The latter are nearly 2 lines apart upon the 
sides of a fragment whose ventro-dorsal diameter is about 8 lines, 
while in a fragment of the body-chamber of another individual 
the first septum is only 1 line distant from the base of the body- 
chamber. The costae do not pass across the periphery, which is 
ornamented only with lines of growth, which form a deep back- 
wardly-directed sinus. A faint longitudinal ridge is seen upon 
casts of the septate part of the shell running along the median line 
of the periphery. This is one of the “ normal lines ” (Normallinie) 
described by G. and F. Sandberger (Die Yersteinerungen des 
Bheinisch. Schichtensyst. in Nassau, 1856, p. 41). 
A small fragment (registered No. C. 3093), consisting of three 
^ Geol. Surv. of Illinois, 1866, vol. ii. Palgeont. p. S86, pi. xxix. ff. 3, 3a, 3h. 
