TRENTON LIMESTONE. 
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138. 2. ORBICULA? FILOSA (». sp.). 
Pl. XXX. Figs. 9 a, b, c, d. 
Orbicular; one valve more or less convex; apex marginal; surface radiated with 
numerous fine elevated threadlike striae, which are more or less prominent, depending on 
exfoliation of the shell; intermediate striae coming in between the others as they recede 
from the beak, but the striae are not bifurcate. 
It is with some hesitation that I refer this shell to the Genus Orbicula, since it presents 
some variation from the usual type. The young specimens are very convex, almost conical, 
but gradually become depressed and expanded as they increase in age and size. 
A single specimen, which is partially exfoliated, presents the marks of three visceral or 
muscular impressions, arranged somewhat like those of the Crania ; but since the shell 
differs as widely from types of that genus, as it does from Orbicula, I should scarcely feel 
authorised to place it under that genus, in the present state of our knowledge regarding it. 
The strongly striated surface presents a deviation from the general character of shells 
belonging to Lingula or Orbicula ; but it is otherwise of the same texture—-black and 
glossy, as those shells. It likewise occurs associated with them, proving that its habit was 
similar, deviating no more from these genera than they do from each other. 
As the shell exfoliates, the surface becomes smoother, and the strife are nearly lost, except 
upon the margins. 
Fig. 9 a. A young shell, very convex or obtusely conical in form. 
Fig. 9 b. A larger shell, less convex than the last : when exfoliated near the beak, the marks of the 
muscular impressions are visible, similar to those of Crania. This portion is magnified. 
Figs. 9 c 6c d. Two apparently full grown shells of this species; the form is much depressed, nearly 
flat, and the surface completely covered by fine striae. 
These specimens bear a close resemblance externally to the Orthis ; but it is easy to 
discover, on examination, that they do not belong to that genus. 
Position and locality . These specimens occur in fine-grained dark limestone, near the 
upper termination of the Trenton mass. They are associated with Orbicula , Lingula , and 
some of the Conularia. Middleville. {State Collection.) 
139. 3. ORBICULA LAMELLOSA (n. sp.). 
Pl. XXX. Figs. 10 a, b. 
Orbicular, depressed ; apex small, but little elevated, situated about one-third the breadth 
of the shell from the margin ; surface marked by elevated lamelliform concentric lines or 
ridges. 
Fig. 10 a. Ventral view. b. Lateral view, 
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