222 BRITISH SILURIAN BRACHIOPODA. 
Ventral valve convex, with a rather deep depression or sinus extending from the frontal 
margin to a short distance from the extremity of the beak. Area narrow and small; fissure 
open, triangular. Dorsal valve moderately convex, with a rather wide, moderately elevated 
mesial fold commencing at a short distance from the umbone, and extending to the 
front. Surface marked by numercus thread-like radiating strize, which increase in number 
as the shell acquires age and size by means of numerous interpolations. In the 
interior of the ventral valve the saucer-shaped muscular depression is considerably bilobed 
or forked, while in the dorsal valve the small cardinal process is situated between two 
deviating, small, curved, projecting brachial processes ; the quadruple muscular scars are 
deep, and longitudinally divided by a rather wide, rounded, mesial ridge. Two specimens 
measured— 
Length 8, width 10 lines. 
pitts amiSsdepthia dines, 
Obs. A glance at figures 11 and 12 of our plate, which illustrate the typical form of 
Salter’s O. reversa, and at the Ayrshire specimen, fig. 17, will, I think, convince the student 
as to the desirability of maintaining this last as at least a well-marked variety of the | 
first. The presence of a mesial fold in the ventral valve, the general shape of the 
shell, and form of the muscular scars in the ventral valve, will warrant our conclusion. 
While describing this shell at p. 261 of his work on British Fossil Brachiopoda, Prof. M‘Coy 
further observes that “the striation of this species is minutely granular under the lens, 
asin the O. elegantula, with which imperfect specimens might very readily be confounded ; 
but the striz are coarser, much more equal, obtuse, and close together, and in that species 
dichotomise, and do not present the subalternate character of the finer striation of this 
species, which is narrower, and very easily distinguished by the broad, flat, mesial depres- 
sion in the receiving (ventral) valve, when that part is seen. Some of the specimens 
show the hinge-line more extended, and the mesial ridge in the entering (dorsal) valve 
defined, leading to the belief that O. fallax (Salter) may be an extreme variety of the 
present species, mainly distinguished by a coarser striation.” 
Position and Locality. At p. 230 of the 2nd ed. of ‘Siluria’ Mr. Salter mentions 
this shell as a characteristic fossil of the Lower Llandovery rocks. In Scotland it occurs at 
Mulloch Quarry, Dalquorhan, and other localities in the Girvan valley, Ayrshire. In 
England it was found in the Woolhope Limestone of Sandbanks, Presteign,—at least a very 
good specimen (fig. 18 of our plate) is so labelled by Mr. Salter in the Museum of Practical 
Geology ; it is also stated to occur in the Lower Llandovery at Cefnmyddan, and in the 
Upper Llandovery at Charfield Green, Tortworth. 
