230 BRITISH PALAXOZOIC FOSSILS. [ Bracutopopa. 
of the shell, and to which the lamellz of the very short thick cardinal teeth seem to converge, forming the 
mesial septum by their union; the wide triangular boss of the foramen slit by a slender rostral tooth, 
Length of small specimen nine lines, proportional length of entering valve 7, width =, depth of receiving 
valve = to +, depth of entering valve =, to =, (length occasionally one inch). 
From the great variation in the proportional depth of the entering valve in different specimens, I am 
uncertain which of the valves is the deepest, although J have little doubt the entering one is so. I have seen 
small specimens confounded with O. e/egantula, from which the characteristic turgid appearance from the 
tumidity of the sides, larger mesial strize, &c. distinguish it. In the adult state the tumid form, and deep 
short furrow in the rostral half of the entering valve, distinguish it easily from all I know. 
Position and Locality —Extremely common in the Bala schists of Craig y beri, Llanarmon-fach, Denbigh- 
shire; Bala schists of Coed Sion, Llangadoc, S. Wales; very common in the Bala schists of Aber Hirnant, 
EK. of Bala, N. Wales; Bala limestone and coarse schists of Goldengrove, Llandeilo, Caermarthenshire; Bala- 
schists of hill half mile N. of Conway Falls; common in the Bala limestone of Llandeilo, Caermarthenshire ; 
Bala limestone of Mathyrafal-fridd, Meifod, Montgomeryshire; sandstone of May Hill, Gloucestershire; Upper 
Bala limestone of Craigwen. 
Explanation of Figures—P\. 1. A. fig. 20. Natural size from the limestone of Llandeilo.—Fig. 20 a- 
Portion of surface magnified.—Fig. 21. Internal cast of receiving valve from Goldengrove—Fig. 22—238, 
Internal casts of entering valve, the former from Craig y beri, the latter from the hills N. of Conway 
Falls—Fig. 24. End view. 
Ortuis VESPERTILIO (Sovw.) 
Ref. and Syn. = 0. bilobata Sow. Sil. Syst. t. 19. f. 7. + Orthis Vespertilio Sow. Sil. Syst. t. 20. f. 11. 
Sp. Ch.—Rotundato-quadrate, hinge-line slightly exceeding the width of the shell, sides subparallel, 
slightly convex, or a little concave close to the cardinal angles, which are thus rendered acute ; front wide, 
obtusely bilobed by a shallow sinus in the middle; entering valve very gibbous, deeply bilobed by a strong 
obtusely angular sinus, dividing the beak and extending thence to the sinus in the margin, which it very 
slightly elevates towards the receiving valve; receiving valve flattened or slightly concave or convex, with 
a very strong, prominent, angular ridge, extending sharply from the beak to the sinus in the front margin; 
cardinal area rather narrow, triangular, flattened, height about one-eighth the width in the receiving valve, 
and inclined backwards at an angle of about 115°, parallel-sided and nearly in the plane of the margins in the 
gibbous entering valve, and rather more than half the height of that of the receiving valve; both valves marked 
with yery numerous, close, sharply angular, radiating ridges, frequently dichotomising ; the intervening sulci 
very variable in strength, which in many specimens produces a strong fasciculation; four or five small sulci 
between each pair of larger, (the larger sulci forming broad undulations on the cast, which is marked by the 
finer ones only near the margin), but in other specimens, or other parts of the same specimen, the suleation 
is nearly uniform; angular interspaces about as wide as the ridges, transversely crenulated, about eight or 
nine strie in two lines at four lines from the beak, or six or seven in the same space at the margin, 
an inch from the beak: internal cast of entering valve having the triangular boss of the foramen divided 
by a large ovate pit of the rostral tooth; cardinal teeth short, triangular, very divergent, forming thick 
ovato-triangular pits close in front of the beak; separated by the deep mesial sulcus, are the two obtuse beak- 
like prominences of the posterior pair of muscular impressions, which have no visible anterior or lateral 
boundaries ; receiving valve with two thick, short, dental lamelle, diverging at an angle of 110°, and a very 
small mesial septum, with a faintly marked radiating muscular impression, reaching one-fourth the length of 
the shell. | Width one inch eight lines, proportional length of either valve =, depth most usually =, but 
occasionally = or =. 
This species varies prodigiously in the amount of gibbosity of the bilobed, entering valve, ranging by 
imperceptible gradation from almost hemispherical to nearly flat: the receiving valve also varies but to a 
less extent; it is always approximately flat at the sides, but varies from convex to concaye, and after the 
