384 BRITISH PALAXOZOIC FOSSILS. [Bracuiopopa. 
Genus. PENTAMERUS. See page 208. 
PENTAMERUS BREVIROSTRIS (Phill. Sp.) 
Syn, = Strigocephalus brevirostris Phill. Pal. Foss. f. 143. 
Sp. Ch.—Suborbicular or very broad-ovate, globose; entering valve nearly orbicular, evenly tumid, with 
an obtuse short beak; receiving valve gibbous, evenly convex, with a rather small, obtuse, incurved beak ; 
beneath which is the usual triangular chamber-like opening, receiving the beak of the entering valve, and bounded 
by the converging lamellze, which unite before reaching the inner surface of the valve into a strong mesial 
septum, reaching nearly to the front margin; in old specimens there is a faintly-marked, narrow, flat, mesial 
ridge; surface smooth. Length one inch, proportional length of entering valve =, width ;*,, depth about *. 
The internal plates of the beak are those of Pentamerus, and not of Stringocephalus, and therefore 
remove the species to the former genus. 
Position and Locality.—Common in the limestone of Plymouth. 
8th Family. ORTHISIDZ. See page 211. 
Genus. ORTHIS (Dal.) See page 212. 
OrTHIS ARCUATA (Phill.) 
Ref—Phil. Pal. Foss. f. 107. 
Sp. Ch.—Transversely elliptical, front wide, slightly convex; hinge-line short, blending with the ellipti- 
cally-rounded sides, depressed; surface radiated with very numerous, subequal, obtuse striz, divaricatingly 
arched backwards towards the ends of the hinge-line, crossed by minute, transverse lines of growth, and dicho- 
tomising at lengths of two lines; thirteen or fourteen ridges in the space of two lines at four lines from the 
beak, about ten in the same space at the margin of a specimen nine lines long. Width of old specimen one 
inch three lines, proportional length {;. 
The wide, regular, transversely elliptical outline, want of the minute interstitial line, and the much finer 
and more equal striz, as well as their backward or divaricating curve on the sides, separate this species from 
the O. interlineata of Sowerby. 
Position and Locality.—Rare in the slates of S. Petherwin. 
OrTHIS INTERLINEATA (So7.) 
Ref. and Syn.—ld. Sow. Geol. Trans. 2nd Series, Vol. V. t. 53. f. 11. and t. 54. f. 14; Phill. Pal. Foss. t. 26. 
f. 106 a, 6. = Atrypa striatula Sow. Geol. Trans. 2nd Series, Vol. V. t. 54. f. 10. 
Sp. Ch.—Suborbicular or rotundato-quadrate when young, transversely oblong or elliptical when old ; 
hinge-line less than the width of the shell; cardinal angles very obtuse ; valves gently convex; margins hori- 
zontal, or with a very slight wave in the front margin ; front slightly convex, wide; sides rounded; both valves 
radiated with yery numerous, subequal, obtusely-angular, radiating ridges, branching di- or tri-chotomously at 
distances of about two lines; ridges separated by narrow sulci, usually containing one or rarely three minute 
elevated lines; eight to twelve of the larger striz in the space of two lines, at four lines from the beak, about 
the same number at the margins of old specimens an inch and quarter long: internal cast of receiving valve 
with two thick dental lamellae, diverging at 115°, from the ends of which an obscure boundary extends forwards 
and then inwards, inclosing a quadrate space for the pair of muscular impressions, nearly one-third wider than 
long, and divided in the anterior half by a slender mesial septum, which does not extend beyond their length : 
muscular impressions extending one half the length of a shell four lines long, about only one-third the length in 
a specimen one inch and quarter long ; cast of entering valve with a small rostral tooth, and two thick, diverging, 
cardinal teeth; two pairs of muscular impressions inclosed by a rounded crucial boundary, as wide as long, the 
middle divided by a broad mesial ridge extending to the front in young specimens. Length of the type- 
specimens about five lines, but adult examples one inch three lines long; the length varies from = to * of the 
width, 4 being the average. 
