RHYNCHONELLIDA.. 191 
broad convex curve ; beak closely incurved over the umbone of the opposite valve. Dorsal 
valve concave, but slightly convex in the contiguity of the hinge-line and sides. Surface 
of both valves smooth, marked only by a few fine lines of growth. In the inner surface 
of the dorsal valve a slightly raised mesial ridge extends from under the hinge-plates to about 
half the length of the valve, and on either side are two scars formed by the adductor 
muscle, while in the internal cast the place occupied by the mesial ridge forms a longi- 
tudinal groove, the muscular impressions being slightly in relief on either side. The 
sockets are widely separate. In the interior of the ventral valve a mesial groove extends 
from the extremity of the beak to about the middle of the shell ; and on either side, running 
parallel with the hinge-line, are two rather broad, rounded projections, at the outer 
extremity of which is situated the articulating tooth ; under these are two obliquely placed 
or chevron-like, elongated, oval-shaped muscular scars, considerably raised from the 
bottom of the valve (these projecting parts forming corresponding depressions on the 
internal cast). 
Length 6, width 43, depth 4 lines. 
Obs. Surely this shell differs much, both by its external as well as itsinternal cha- 
racters, from those peculiar to the genus RAynchonella ; so much so that it may hereafter 
be found desirable to propose for it and similarly characterised shells a separate generic or 
sub-generic designation. Not being, however, quite satisfied as to the shape and means 
by which its oral arms were supported, I have preferred to follow the greater number of 
paleontologists, leaving it provisionally with Rhynchonella. Prof. M‘Coy justly ob- 
serves that “the peculiar chevron mark on the cast of the greater valve is very remark- 
able, and makes the cast very easily recognisable; that it forms the anterior boundary 
apparently of two great prominent muscular impressions, which are marked by radiating 
strize from the beak :”” and, as Mr. J. de C. Sowerby observes, its ‘‘ boat-shape” is very pecu- 
liar. The shell does not seem to have much exceeded the proportions above given. 
Position and Locality. Rhynchonella ? navicula appears confined to the Wenlock 
and Ludlow formations, and is more abundant in the last mentioned. In the Upper 
Ludlow it occurs near Ludlow, at Usk Castle; Hale’s End and Overley, in the Malvern 
district ; Pyrton Passage, Tortworth district; Cwm-Craig-ddu, in the Builth district. 
Mr. J. de C. Sowerby mentions the following localities:—Ludlow Promontory; Clyro 
Hill, Radnorshire; Brecon Anticlinal, viz. Corn-y-Tan, Alltfawr, and Rhiwannest. In 
the Aymestry Limestone it is common at Sedgley, near Wolverhampton, also at Llanbadoc, 
in the Usk district, &e. In the Lower Ludlow at Cefn Ila. Inthe Wenlock Shale near 
Builth (Siluria). Mr. Salter mentions Cefn Barog; Moel-Todig; Dinas Bran; Welshpool, 
Mynydd-y-Gaer ; Llanefydd, in North Wales. Other localities are enumerated by Profs. 
M‘Coy and Phillips ; but from these I have not seen specimens. 
In Ireland it occurs in the Ludlow and Wenlock Shales at Doonquin and Fer- 
riter’s Cove, Dingle, County Kerry, &e. It has not yet been recorded as a Scottish fossil. 
Abroad it is found in the Island of Gothland; in Bohemia, &c. 
