294 TRENTON FOSSILS. 



are distinctly in fascicles of four to six, separated by stronger and more elevated single ones on either side; 

 as the shell increases, this uniformity in the arrangement often disappears, and the 

 strife appear of uniform size, or alternate irregularly. The concave or ventral valve, 

 particularly in old specimens, has the strife nearly uniform in size. 



Fig. 199 represents rather a small specimen of Strophomena alternata, but one 

 which presents the characters of the species very well. The observer will notice the 

 elevated strife, and the gradual enlarging of some of the striae as they recede from the 



apex. This very common species has been collected at all the prominent localities of 



Strophomena alternata. Trenton fossils, viz; at Larrabee's Point in Shoreham, Frost's landing in Bridport, 



McNeil's Point, at the north end of Isle La Motte, and in S. Hero and Grand Isle. 

 Leptcena is perhaps a subgenus of Strophomena. It has the valves regularly curved; the dorsal 

 are concave, thickened, and the muscular impressions are elongated. 



Leptcena sericea (Sowerby), is the only species of this genus yet found in Vermont. It is semi-oval : 

 length usually somewhat more than half the width on the cardinal line; cardinal line extended a little 

 beyond the sides of the shell; dorsal valve convex in the center, deflected at the edge; ventral valve concave 

 (nearly flat Sow.) ; surface marked by fine striae, which are even and uniform, or alternating with stronger 

 ones; strife increasing in number towards the margin, granulate or papillose, crossed by a few lines of 

 growth; surface shining. 



This beautiful and abundant little shell is readily distinguished by its almost perfectly semi-oval form, 

 with fine papillose stria3 alternating with larger ones; the latter are often obsolete, and the surface appears 

 uniformly striated. 



Fig. 200 represents one valve of the Leptcena sericea, upon which appear equal stria? and a few concen- 

 tric lines of growth. This species is found in great abundance in the Trenton limestone, at 

 Larrabee's Point in Shoreham, at McNeil's Point in Charlotte, at Grand Isle, at South Hero, 



and at the north end of Isle La Motte. Some of the specimens from Larrabee's Point are 

 the finest, because the shells have been partly calcined by exposure to the heat of a furnace. 

 Orthis. The shell of the Orthis is transversely oblong, radiately striated or plaited, bi- 

 convex, hinge line narrower than the shell, cardinal process simple, brachial processes tooth-like, prominent 

 and curved. 



The 0. testudinaria, (Dalm), is suborbicular, plano-convex; cardinal line straight, shorter than the 

 width of the shell; dorsal valve convex, much elevated towards the beak, often with an elevated ridge 

 down the center; beak small, slightly incurved; ventral valve flat, or with a longitudinal depression along 

 the center, which often produces an emargination in front; cardinal area small; foramen small, triangular; 

 surface covered with fine striae, which bifurcate towards the margin, and are crossed by elevated thread-like 

 lines, giving them a crenulated appearance. 



This species is somewhat variable. Most of the specimens from the north-east have a regularly depressed 

 convex dorsal valve, with a flat or sometimes slightly convex ventral valve; while shose from the west have 

 very frequently an elevated longitudinal ridge on the dorsal valve, and a depression along the flat or ventral 

 valve. 



Fig. 201 is intended to represent the common appearance of the Orthis testudinaria. This species is 

 found in all the localities in which the preceding species occurs. The Orthis testudinaria, 

 and Leptcena sericea are the most common fossils of the Trenton limestone. 



Orthis pectinella (Con.) The shell of this species is suborbicular or obtusely semi-oval, 

 wider than long in the proportion of about 9 to 12 ; cardinal line extended, equal to or less 

 than the greatest width of the shell, slightly deflected at the extremities ; area moderately 

 large and well defined; shell resupinate, or the area and foramen being principally on the 

 flatter side, or partially common to both; dorsal valve sub convex near the beak, with flat 

 sides and a broad depression along the center, which is distinct in front; ventral valve regularly convex 

 most prominent in the center; beak extending only to the cardinal line; surface marked with from 22 to 30 

 prominent rounded radii, which are equal to the spaces between; radii simple, or bifid and trifid toward 



