ORTHID. 237 
portions of the valve close to the extremities of the hinge-line are flattened ; hinge-area 
narrow. Ventral valve convex at the beak, but becoming slightly concave towards the 
front and margins, while a strong prominent mesial fold or sharply angular ridge extends 
from the extremity of the beak to the front; area triangular, fissure open. Surface of 
valves marked with numerous small angular radiating striz, often dichotomising or 
increasing in number by the interpolation of additional ribs, which are at times arranged 
in clusters of four or five of unequal length and breadth. In the interior of the dorsal 
valve a projecting cardinal process is situated between two smallish deviating brachial 
lame. In the interior of the ventral valve the saucer-shaped muscular area is 
margined by a slightly raised ridge. 
Length 15, width 19, depth 5 lines. 
Obs. As justly observed by Prof. M‘Coy, ‘This species varies prodigiously in the 
amount of gibbosity of the bilobed dorsal valve, ranging by imperceptible gradation from 
almost hemispherical to nearly flat; the ventral valve also varies, but to a less extent; it 
is always approximately flat at the sides, but varies from convex to concave.” Much 
difference is likewise observable in the respective width of the shell in different indi- 
viduals. ‘I'he larger number of specimens are widest at the hinge-line, while others are 
widest at about half the length of the shell. In 1848 Messrs. Phillips and Salter 
correctly united O. vespertilio and O. bilobata into a single species. 
Position and Locality. O. vespertilio is abundant, and may be said to be characteristic 
of the Caradoc or Bala formation, for the fossil has been but sparingly found in the Lower 
Llandovery. In England it was first discovered by Sir R. Murchison in the Caradoc 
Sandstone of Shropshire, at Acton Scott, &c.; it is common at Harnage, south-east 
of Shrewsbury; Gretton, near Cardington, &c. According to Messrs. Ramsay and 
Salter it is plentiful at Dolwyddelan; Penmachno, Caernarvonshire; south of Llan- 
gollen; south-east of Cerrig-y-Druidion, Denbighshire; Meifod; Welshpool; north 
of Llangedwyn; Llanwddyn; Llanfyllin, Montgomeryshire ; east, west, and south-east of 
Bala Lake; near Corwen; Hirnant Ridge, Merionethshire. It has also been met with 
in the Malvern, May Hill, Llandeilo and Haverfordwest districts, by Prof. Phillips. Prof. 
Harkness has sent me specimens from the Bala Limestone of Keisley, as well as from the 
Lower Bala series or black shales at Pusgill, near Dufton, in Westmoreland. It is also 
said to be a common species at Coniston. 
From the Lower Llandovery I have seen specimens, found by Mr. Hughes one mile 
south of Cwmrhyddan. 
In Scotland it is a very common shell in the Caradoc Limestone at Craighead Quarry, 
near Girvan, in Ayrshire. 
In Ireland it has been found in a number of Caradoc localities, such as at Portrane, 
County Dublin ; Desertcreat, Killey, Pomeroy, County Tyrone, &c. 
It is also stated to occur in Russia. 
