SILURIAN FAUNAS. 239 
The best-preserved brachial valve is somewhat smaller: breadth, 14 
mm., and length, 8 mm. 
Remarks.—Although the punctate shell structure has not been de- 
tected in any of the specimens of this species which have come under 
observation, it has all the outward characters as well as the strong 
median septum in the pedicle valve, of the genus Cyrtina, and there 
seems to be no doubt as to its correct generic reference here. This 
species differs from the only other American Silurian species of the 
genus, C. pyramidata H., in its larger size and in its concave and less 
sharply-defined cardinal area. From any of the Devonian members 
of the genus it may be distinguished by its coarser plications and by 
its less sharply-defined cardinal area. 
RETICULARIA BICOSTATA (Vanuxem). 
Plate XXI., Figs. 4445. 
1842. Orthis bicostatus Van., Geol. N. Y., Rep. 3d Dist., pp. 91, 94. 
1852. Spirifer bicostatus Hall, Pal. N. Y., vol. II., p. 263, pl. 54, 
fig. 4. 
HSIs. Spuifen bicostatus He & ©. Pal Naw vol. Vil: pt. Ei pl: 
36, fig. 7. 
1897. Reticularia bicostata Schuchert, Bull. U. S. Geol. Surv., No. 
87, p. 341. 
Description.—Shell transversely subelliptical, the hinge-lne less 
than the greatest breadth. Pedicle valve strongly convex, beak promi- 
nent and incurved, cardinal area arched, not sharply defined, mesial 
sinus shallow, without plications. On each lateral slope of the valve 
are three or sometimes four, low, rounded, inconspicuous plications, 
which do not reach to the beak; the largest one of these bounds the 
mesial sinus, while the others successively grow smaller. Brachial 
valve less convex than the opposite one, with a low, rather sharply- 
defined mesial fold, and with indistinct plications on the sides of the 
valve corresponding with those of the pedicle valve. The surface of 
both valves marked by moderately fine, concentric, lamellose lines. 
The dimensions of a large pedicle valve are: length, 13 mm.; 
width, 16 mm.; convexity, 6 mm. 
