SILURIAN FAUNAS. 23T 
brachial valve. Most of the Helderbergian representatives of the 
species are the more finely-marked forms, but in the Silurian faunas 
both coarsely and finely-marked varieties occur. 
ATRYPA? LAMELLATA Hall. 
Plate XXI., Figs. 23-29. 
1852. <Atrypa lamellata Hall, Pal. N. Y., vol. II., p. 329, pl. ¥4, 
fig. 11. 
1897. Rhynchonella? lamellata Schuchert, Bull. U. S. Geol. Surv., 
No. 8%, p. 359. 
Description.—Shell small, subrhomboidal in outline, the valves sub- 
equally convex. Pedicle valve with an acutely-pointed, incurved beak ; 
the sharply-defined mesial sinus deep in front and flattened in the 
bottom, not continuous to the beak. Brachial valve with an elevated 
fold in front, the lateral slopes regularly convex. Surface of each 
valve marked by from ten to fourteen simple, rounded or subangular, 
radiating plications, one, two or three, but usually two, of which are 
within the mesial sinus of the pedicle valve, with a corresponding 
number on the fold of the opposite valve. The radiating plications 
are crossed by strong, imbricating, concentric lamelle, which give 
to the whole surface of the shell a rugose appearance. 
The dimensions of a large specimen are: length, 7 mm.; width, 
7.25 mm.; thickness, 5 mm. Another specimen measures: length, 
5.66 mm.; width, 7 mm.; thickness, 4 mm. 
Remarks.—Most specimens of this species are smaller than those 
whose dimensions have been given. ‘The shell has a strong resem- 
blance to those members of the genus Atrypa which are allied to A. 
marginalis Dal—in fact, this species resembles in many respects a 
diminutive A. marginalis. In order to determine whether or not a 
brachidium of the type of that in the genus Afrypa is present in 
this species, numerous specimens have been ground down, but in no 
case have the spires been observed, but a pair of long, slender crura 
have been detected. ‘The generic reference of the species, therefore, 
remains in doubt, and it is quite possible that it should be placed wi 
the Rhynchonelloids, as has been done by Schuchert. 
