BRACEIOPODA. 
203 
culiar. The shell is transversely oval, with a straight hinge-line and rounded 
cardinal extremities; its form is depressed plano-convex, the convexity of the 
pedicle-valve being slight. The cardinal area is about equally developed on 
each valve. The surface is ornamented by fine, rounded, radiating striae be¬ 
tween each two of which are from two to six much finer radiating lines; all 
these are crenulated by exceedingly faint growth-lines. 
The interior of the pedicle-valve has short teeth with prominent extremi¬ 
ties, and inconspicuous dental lamellae. The muscular impression is large and 
consists of a small adductor scar situated centrally, and two lateral scars which 
are flabellate and greatly elongated, but not uniting in front and enclosing the 
adductor as is usual in typical forms of Orthis. These lateral scars are divided 
into subordinate impressions, of which the outer posterior members may rep¬ 
resent the adjustors. In the brachial valve the dental sockets are obscure, the 
crural plates oblique, terminating abruptly at the bases of the crura, their 
lower part being continued in a low ridge surrounding a short subcircular 
muscular area, which is very obscurely quadrilobate. The cardinal process 
has a vertical, sharp, simple, posterior edge, but is much thickened where it 
unites with the crural plates, and is produced along the muscular area as a 
prominent median ridge. From the ante-lateral margins of the muscular area 
radiate six low, somewhat sinuous ridges of similar character to those frequently 
seen in members of Group XI. Both valves are considerably thickened just 
within the margins, as in many strophomenoids. 
The shell-structure is finely fibrous and perforated by minute punctations 
which are in general sparsely developed, but most distinctly arranged in 
radiating rows corresponding to the surface striae. 
Orthis Clytie is the only representative of this type of structure known in the 
American Silurian, but with it may be associated Sowerby’s 0 . alternata* and 
McCoy’s f 0 . retrorsistria, both from the Caradoc horizon. The former of these 
is a very close ally to the American Trenton form in all its external specific 
characters. 
* Sowerby, in Murchison’s Silurian System, p. 638, pi. xix, fig-. 6 ; 1839, and Davidson, Silurian Braehio- 
poda, p. 264, pi. xxxi, figs. 1, 3, 7; Suppl. p. 1S7, pi. xiv, figs. 1-6. 
f McCoy, British Palaeozoic Fossils, p. 224, pi. i h, figs. 12,13; 1852, and Davidson, Silurian Brachiopoda, 
p. 265, pi. xxxi, figs. 2, 4-6; pi. xxxvi, figs, 39-42; Suppl. p. 185, pi. xiv, figs. 7-16. 
