Lorraine Faunas of New York and Quebec 
269 
12. Caritodens demissa, Conrad 
{Plate I, Fig. 10; Plate III, Fig. 11) 
The specimen of Pterinea demissa here figured, from the Riviere 
des Hurons, presents the following characteristics. With in- 
creasing age the sinuosity of the anterior margin of the shell be- 
comes more pronounced, the angle between the anterior margin of 
the ear and body increasing from less than 25 to frequently more 
than 55 degrees. This is due chiefly to a retardation of growth 
of the anterior margin of the body below its junction with the ear, 
and an acceleration of growth along the basal margin. A similar 
retardation of growth takes place along the line of junction of the 
posterior margin of the body with the wing. The result is that 
the anterior margin of the body becomes more erect, the shell is 
strongly prolonged in the direction of the basal margin, assuming 
a more oblong form, and the general appearance is that of a less 
oblique shell. In the meantime there is a considerable prolonga- 
tion of the shell along the hinge-line, both anteriorly and poste- 
riorly, these parts, usually, however, not being well preserved. 
In the fine-grained Lorraine sandstones, near Vars, east of Ottawa, 
the surface ornamentation is preserved in the form of fine, sharp, 
concentric striae, separated by much broader, flat, concentric 
spaces. Judging from well preserved specimens found in clays 
of the Richmond formation, on the Nicolet River, the sharp 
striae may have served as lines of support for narrow lamellose 
extensions of the shell, 0.5 mm. or less in width. 
The shell substance evidently consists of two layers, of which 
the outer layer is very thin, rarely more than 0.25 mm., and often 
less in thickness; the inner layer frequently attains a thickness of 
1 mm., and sometimes exceeds even 2 mm. in thickness. Speci- 
mens preserving both the outer and inner layers are common in 
the Richmond beds along the Nicolet River, several miles south- 
east of Ste. Monique, in the province of Quebec. In these speci- 
mens, the thick, inner layer appears to have undergone secondary 
crystallization ; it is whitish in color, of loose texture, and probably 
dissolved readily under most conditions attending fossilization, 
since this layer so rarely is seen; its outer surface, where overlaid 
by the thin outer layer, is smooth, not showing the concentric 
striae belonging to the outer surface of the outer layer. The 
