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- - SILURIAN FAUNAS. 
ORTHOTHETES INTERSTRIATUS (Hall). 
Plate XX., Figs. 8-9: 
41852. Orthis interstriata Hall, Pal. N. Y., vol. LI., p. 326, pl. 74, 
figs. 1 a—b, 2.a-b. 
1897. Orthothetes interstriatus Schuchert, Bull. U. S. Geol. Surv., 
} No. 87, p. 297. 
Description.—Shell small, subplano-convex, wider than long, the 
hinge-line straight, about equal to the greatest width of the shell. The 
lateral margins meet the cardinal line at nearly right angles and are 
then regularly rounded to the front of the shell. Pedicle valve 
obscurely subcarinate, its greatest depth near the prominent, suberect 
beak: cardinal area rather high, nearly flat. Brachial valve depressed 
convex or nearly flat, with a broad, shallow, ill-defined mesial sinus. 
Surface of both valves covered with rather coarse, angular, radiating 
coste, which increase by implanation toward the margin, and by rather 
coarse, concentric lines of growth. 
The dimensions of a pedicle valve are: length, 5.5 mm.; width, 
Y mm.; convexity, 1.5 mm. A brachial valve measures: length, 4.5 
mm.; width, 6 mm. 
Remarks.—All of the specimens of this species which have come 
under observation are more or less exfoilated, so that the concentric 
markings are usually wholly obliterated. On a few specimens, how- 
ever, they may be detected. 
ORTHOTHETES DECKERENSIS Nl. Sp. 
Plate XX., Figs. 6-7. 
Description.Shell subplano-convex, transversely subelliptical in 
outline, the hinge-line shorter than the greatest width of the shell, 
which is near the middle. Pedicle valve depressed, the umbonal 
region usually irregularly concave; the concave area, having been a 
facet of attachment to some external object, retains the reverse of the 
contour of the object to which the shell was attached. Between the 
umbonal concavity and the margin this valve is marked by one or 
more conspicuous, more or less irregular, concentric wrinkles, with 
