43 



ribs, with from one to three smaller, and obscure ones next the margin, 

 in the upper half of the shell. The dorsal valve has two large median 

 ribs, with six or seven on each side ; those next the margin in the upper 

 half small and obscure- When the shell is preserved the ribs are rounded, 

 rarely sub-angular. They are crossed by obscure concentric lines, 

 sometimes with one or more wrinkles of growth. 



When the shell is completely absent, as is often the case in the sand- 

 stone, the ribs, as seen on the casts of the interior, are angular, while 

 those of the dorsal valve do not often pass upwards beyond the mid-length 

 of the shell. In the cast of the ventral valve the median rib becomes 

 flattened or obsolete, in the upper half, where there is to be seen a small 

 groove with a small semi-circular scar on each side (fig. 6a.) In the casts 

 of the dorsal valve there is an irregular excavation, in the place of the 

 beak and umbo, with a small medial groove extending down from it and 

 two small punctures on each side. (Fig. 6). 



In the description of this species, published in the Pal. N.Y. vol. 3, it 

 is said that the beak of the ventral valve is incurved, " with a small 

 round perforation in the extremity, which is completed on the lower side 

 by two deltidial pieces ; or, in the absence of these, the foramen is 

 completed by the umbo of the dorsal valve. Our specimens are all 

 imperfect at the beak, and do not exhibit the structure here alluded to. 

 The dorsal valve is also said to be flat. None of the Canadian specimens 

 have (strictly speaking) a flat dorsal valve. It is often flat or nearly 

 so in the upper half, but alwayd more or less convex in the front half, the 

 shell being usually more strongly curved down around the front margin 

 and along the lower half of the sides. In this respect it agrees with 

 L Frimbriata, Hall, (Op. cit., p. 450) which is said to have the dorsal 

 valve flat or slightly convex, the front and lower lateral margins abruptly 

 inflected. 



Length of a large specimen, eight Hnes ; width, four lines. The pro- 

 portions vary within a small limit. 



Under certain conditions of preservation, the shell has a whitish 

 shining lustre, like that often presented by Atrypa reticularis. 



Locality and Formtion. — Gasp^, in No. 8, and also in vast abundance 

 in the lower part of the sandstones ; it occurs also at Percd. Prof Hall 

 cites New York, Maryland, Virginia and Canada ; Oriskany sandstone. 



Collectors,— W. E. Logan, Prof. J. W. Dawson, R. Bell. 



