84 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 8, 
margined by a raised edge, under which are placed the muscular 
(adductor) elevations or scars, which are longitudinally separated 
by a narrow concave space. Length 11, breadth 14 lines. 
The internal cast of the dorsal valve was, in 1863, described and 
figured by Mr. Salter, the exterior and interior of the other valve 
being unknown to him at the time; subsequently, however, im- 
pressions of the exterior, as well as the internal cast of the ventral 
valve, were found by Mr. Vicary, so that it is now possible to draw 
up a complete description of the species. The shell described by 
Mr. Salter as Porambonites, sp.? (from the imperfect internal cast of 
a dorsal valve), appears to me to be a large internal cast of the shell 
under description, in which the muscular impressions are imperfectly 
defined ; but I make this suggestion with reserve. The figure given 
by Mr. Salter is likewise not entirely correct. In the original spe- 
cimen the cardinal angles are more rounded; and the muscular im- 
pressions, &c., are less sharply defined on the cast than they are in 
the figure; nor does it, or Mr. Sharpe’s Portuguese specimens, to 
which it is assimilated, belong to the genus Porambonites. 
T do not know whether it is a Silurian or a Devonian species, as no 
other known form has been found associated with it in the same 
pebble. 
27. Ortuis VicaRyI, n. sp. Pl. V. figs. 20-22. 
Shell circular or elongated oval, slightly wider anteriorly ; hinge- 
line straight, about half the breadth of the shell. Ventral valve 
moderately convex, beak slightly incurved, area narrow and small, 
divided in the middle by a rather wide fissure. Dorsal valve rather 
less convex than the opposite one. Surface of both valves marked 
by numerous fine thread-like radiating lines or raised striee, which 
increase in number by means of numerous interpolations. In the 
interior of the ventral valve two large, deep, oval-shaped muscular 
depressions occupy fully two-thirds of the length of the valve, and 
are divided in the middle by a narrow, angular, slightly project- 
ing ridge. In the interior of the dorsal valve a small, bilobed car- 
dinal process is situated between the prominent, diverging, slightly 
curved brachial processes ; while a rather wide, rounded mesial ridge 
extends from under the cardinal process to about half the length of 
the valve, and separates into two parts the muscular (adductor) 
impressions. ‘Two specimens measured :— 
Length 7, width 7 lines. 
” 5 ” 4 ” 
Of this species several examples have been found by Messrs. Vicary 
and Valpy. It occurs most frequently in the shape of internal casts ; 
but impressions of the external surface are occasionally met with. 
It is remarkable on account of the large proportions of the elongated, 
oval-shaped muscular depressions in the ventral valve. 
Along with this species is found a small Rhynchonella?, which, if 
AR. ovalis, would perhaps indicate that the Orthis under description 
is Silurian; but the question of age will require to remain for the 
present undetermined. 
