270 
Aug. F. Foerste 
outer layer is dark in color and dense in texture. This is the 
layer most frequently preserved, when any part of the shell is 
present. Shells in which the thick inner layer has disappeared 
usually do not present any trace of the inner surface of the valves 
so that the inner surface has long been unknown. Many strange 
things, however, may happen during the processes of fossilization. 
In the case of Pterinea demissa, when found in fine-grained sand- 
stones, it occasionally happens that the rock, when split open, 
exposes on one piece the impression of the exterior of a valve, and 
on the other piece the impression apparently of the interior; but 
only the most pronounced features of the interior are retained, 
usually the crural ridges forming the jugum, to be described later, 
and the inner outline of the ligamental area. Superposed upon 
this cast of the inner surface of the valve are traces of the concen- 
tric, striate structure belonging to the outer surface of the valve, 
especially along the body of the valve. 
The interior of the left valve, in one specimen from the Rich- 
mond strata along the Nicolet River, is characterized by a longi- 
tudinally striated ligamental area, 2.5 mm. in height near the 
beak. About 2 mm. below the lower edge of this ligamental area, 
a short distance posterior to the beak, the inner edge of the jugum 
is located. This is a lunate callosity or thickening of the interior 
of the shell a short distance below the ligamental area, the concave 
side facing the middle parts of the valve, the posterior extension 
curving away from the ligamental area, following approximately 
the line of junction between the body of the valve and the posterior 
wing, while the anterior crural ridge or anterior part of the jugum 
extends for only a short distance along the line of junction between 
the body of the valve and the anterior ear. This jugum is that 
part of the inner surface of the valve most commonly indicated 
in casts of the interior of the shell. Between the inner edge of the 
jugum and the lower margin of the ligamental area the shell is 
thickened. Transverse to this thickening, below the beak, there 
are several ridges serving as anterior cardinal teeth. In fact, the 
posterior extension of the jugum serves as a posterior lateral tooth. 
^ Nothing similar to this structure is known in the case of Glypto- 
desnia. The nearest approach is seen in the shell described by 
Hall as Pterinea flabella from the Hamilton group of New York 
{Paleontology of New York, vol. V, part I, plate XIV, Fig. 19, and 
plate XV, Fig. 5). The posterior muscular scar is large and lies 
