TRENTON LIMESTONE. 99 



138. 2. ORBICULAI FILOSA («. 5;3.)- 



Pl. XXX. Figs. 9 a, b, c, d. 



Orbicular ; one valve more or less convex ; apex marginal ; surface radiated with 

 numerous fine elevated threadlike stria;, wliich are more or less prominent, depending on 

 cxfolialioii of the shell; intermediate stria; coming in between the otlmrs as they recede 

 from the beak, but the stria; 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 tbat 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 striae are nearly lost, except 

 upon the margins. " ^ .. 



Fig. 9 a. A young shell; very convex or obtusely conical in form. 



Fig. 9 6. 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. 2 c &, d. Two apparently full grown shells of this species; the form is much depressed, nearly 



flat, and the surface completely covered by fine stria;. 



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. Tlipy are associated with Orbicula., Lingula, and 

 some of the Conularia. Middleville. (*''«'« Collection.) 



139. 3. ORBICULA LAMELLOSA ( n. .7;.). 



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. ' 



13' 



