448 BRITISH PALAZOZOIC FOSSILS. [ Bracntopopa. 
the sides, flattened in the middle; hinge-line rather less than the width of the shell; cardinal angles obtuse ; 
lateral and front margins nearly in one plane in young specimens ; in old individuals the lateral margins are 
very slightly sigmoid, and the middle of the front raised into a small, narrow, rotundato-quadrate sinus, from 
which a slightly elevated flattened ridge extends a short way on both valves of most specimens, but often obso- 
lete. Receiving valve with a moderate, slightly curved, triangular area, nearly at right angles to the plane of the 
lateral margins; beak slightly tumid, prominent; surface sloping to the margins with slight convexity for about 
nine or ten lines, after which the margin is obtusely deflected nearly at right angles. Entering valve much 
deeper than the receiving one, beak large, obtuse ; whole surface moderately convex for nine or ten lines, after 
which the margin is obtusely deflected nearly at right angles, the rounded intervening angle being the deepest 
part of the valve in old specimens; greatest depth of the rostral portion, at about one-third from the beak. 
Surface with very fine, nearly regular, longitudinal striz (averaging twenty in two lines at six lines from the 
beak), some of them at irregular intervals abruptly thicker than the rest; concentric lines and imbrications of 
growth slightly marked. Width of adult specimen one inch three lines, proportional width of hinge-line {;, 
height of cardinal area ;3, length of moderately convex rostral portion of each valve ;;, depth of deflected front 
of entering valve to side of mesial sinus ;j;, depth of deflected front of receiving valve to same point ;;, width 
of sinus in front margin 7, depth of do. ;j;. 
The beak of the entering valve is slightly more prominent than that of the receiving one, if measured from 
the front margin. The very great gibbosity of the adults, and the extremely fine striation, separate this species 
from either the O. connivens, or O. resupinata; the inflated portions of the striz are not placed so regularly 
alternate as in the O. resupinata, nor are they so rapidly thickened nor truncated after so short a length; but 
instead of the triangular, drop-like appearance which they present in that species, they rather resemble thickened 
strize continued for a great length, often with considerable regularity. 
Position and Locality—One obscure specimen from the lower carboniferous limestone of Kendal, West- 
moreland. 
Ortats Micue ini (Lév. Sp.) 
Ref. and Syn. = Terebratula id. Léveillé Mém. de la Soe. Géol. de France, Vol. II. t. 2. f. 14 to 17 = Spirifera 
filiaria Phill. Geol. York. Vol. II. t. 11. f. 3 = Orthis Michelini M. V. K. Geol. Russ. t. 13. f. 1. 
Desc.—Longitudinally subtrigonal or obovate ; greatest width a little behind the front margin, equal to the 
length; front broad, nearly straight ; lateral margins long, very slightly convex, converging posteriorly to the 
very narrow hinge-line, depressed ; lateral and front margins nearly in one plane, without distinct sinus. Entering 
valve deeper than the receiving one, gently convex along the middle; beak very small, depressed, slightly pro- 
minent; profile gently arched, greatest depth at one-third from the beak; a very faint, wide, mesial flattening or 
concavity extends nearly from the beak to the front margin; sides rapidly sloping with gentle convexity to the 
margins. Receiving valve rather tumid near the beak, which is of moderate size and prominent; profile sloping 
from a little in front of the beak, with very slight convexity, to the front margins; sides flattened, abruptly 
sloping to the margins from the middle portion, which is wide and flattened after a few lines from the beak. 
Surface of both valves marked with rather coarse irregular imbrications of growth, radiated by rather coarse 
subequal strize of nearly uniform size over the whole shell (usually ten in two lines at six lines from the beak), 
occasionally dichotomising at irregular distances, and at moderately rare intervals set with short, solitary, 
pointed tubercles, nearly as thick as the ridges on which they are placed (but not preceded by any remarkable 
thickening, nor followed by any marked diminution of the ridge, as in O. reswpinata) ; granulo-punctation under 
the lens, moderately coarse. Cardinal area of entering valve very small, in the plane of the lateral margins; area 
of receiving valve somewhat larger, inclined at about 40°. Average length and width eleven lines, length of 
entering valve slightly less, proportional depth of receiving valve ;,, of entering valve ;;;, length of hinge-line =, 
depth of area about ;%, (one specimen one inch six lines long). 
The form of this species is very peculiar, from the greatest width being near the wide subtruncate front 
margin, and the posterior end seeming narrowed, from the long slightly curved sides converging to the very 
