Bracuioropa. | LOWER PALAZOZOIC MOLLUSCA. 199 
of Keeper’s Lodge, Goldengrove, Llandeilo, above second grits; common in the Wenlock limestone of Wool- 
hope; Upper Ludlow of Brockton and Burton, near Wenlock ; Woolhope limestone of Littlehope; fine 
sandstone of May Hill. 
7th Family. RYNCHONELLAD. 
Arms free in their entire length, rolled laterally, and sustained by two separate, internal, arched, shelly 
apophyses, and capable of extension beyond the shell, provided with short fringes; texture of the shell fibrous, 
not punetured,—an opening under the beak of the large valve, which is pointed. 
Genera: 1, Hemithyris (V@Orb.); 2, Rhynchonella (Fisch.); 3, Strigocephalus (Def.) ; 4, Pentamerus 
(has an opening, and ought to come here); 5, Camerophoria. (The genus Porambonites, placed in this 
family by d’Orbigny, seems from its double ares to be better placed in the family Orthisidw, where I have 
arranged it). 
Genus. HEMITHYRIS (d’Or?.) 
Gen. Char.—Foramen triangular, not separated from the hinge; beak acute, pointed, entire; no cardinal 
area; entering valve with a small mesial septum; apophyses short, arched, triangular, with small dental 
lamellz ; two strong, diverging, cardinal teeth bordering the opening in the large valve, supported by dental 
lamellee, extending to the surface of the valve, (type, Zerebratula psittacea). 
The fossil species even of the Paleozoic Rocks seem to agree exactly with the recent type in fibrous 
tissue of the shell, form of the beak and foramen, internal dental lamellze in beak of receiving valve (leaving 
two diverging slits in the cast), &c. They shew great variety in the size of the opening, depending on the 
degree of curvature of the beak, which sometimes seems to close it entirely ; casts usually shew two elongate 
muscular impressions close to the mesial line on the rostral half of the entering valve, and these being bounded 
by slight ridges, we have one or three slight longitudinal sulci on the cast ; something similar may be observed 
on old specimens of the recent H. psittacea. 
Mr Davidson refers this genus to Hypothyris of Phillips, but a reference to the ‘‘ Palzeozoie Fossils” shews 
that that genus has the opening separated from the hinge, and is therefore equal, as d’Orbigny shews, to 
Rhynchonella (Fisch.) It is Cliothyris of Phillips (proposed as a substitute for Atrypa) that includes the fossil 
species with foramen touching the hinge, along with the closed Atrypw ; and I should have considered this 
latter name the proper one for this group, but for Prof. Phillips, to my great surprise, placing the Silurian 
species in his genus Hypothyris, in the 2nd Vol. of the Memoirs of the British Geological Survey. Under 
these circumstances then, there seems no alternative but to adopt M. d’Orbigny’s clearly defined and extensively 
applied genus. 
HEMITHYRIS ANGUSTIFRONS (J/‘Coy). Pl. 1. H. fig. 6 to 8. 
Ref —ld. Quart. Geol. Journ. y. 7. t. 9. f. 10; M°Coy, Annals Nat. Hist. 2nd Series, Vol. VIII. p. 391. 
Sp. Ch.—Elongate ovate; both valves very gibbous in the middle, gradually sloping to the margins, 
which are nearly level, or with a very slight elevation of the front towards the entering valve; greatest 
width in most specimens rather behind the middle, from whence the width diminishes to the narrow, obtusely- 
rounded front; beak of the receiving valve large, pointed, with a long triangular opening beneath it; sub- 
stance of the shell coarsely fibrous ; surface nearly smooth, or with irregular transverse squamz of growth: 
casts shew in the entering valve two sub-parallel, approximate, longitudinal sulci, marking the inner edges 
of the muscular impressions, and with a fainter sulcus between them, left by the slight mesial septum; two 
pits near the beak left by the apophyses strong; receiving valve with two strong dental lamellie, one each 
side of the beak, and a slight indication of a mesial septum; few straight, once- or twice-branched impres- 
sions of the pallial vessels on each side. Length nine lines, proportional length of entering valve 5, width 
%, depth of both valves *; shell fibrous. 
The general ovate form of this species with its narrow front, without mesial ridge or sinus, and its 
pointed beak with large foramen, easily distinguish it from any of the other Lower Paleozoic forms. It 
