Bracuiopvopa., | LOWER PALAZOZOIC MOLLUSCA. 231 
careful examination of a very extensive suite of fine specimens, I am accordingly convinced of the specific 
identity of the O. dilobata and O. Vespertilio of Sowerby, as suggested by Prof. Phillips and Mr Salter in 
the Memoirs of the Geological Survey. 
Position and Locality—Very common in the fine Bala slates of Blain y Cwm, W. of Nantyre, Glyn 
Ceiriog, Denbighshire ; common in the fine Caradoc sandstone of Acton Scott, Shropshire; very common in 
the Bala sandstone of Alt yr Anker, Meifod, Montgomeryshire; very common in the Bala schists of Bryn 
Melyn, near Bala, Merionethshire ; common in the Eala schists of Das Hithen ridge, Hirnant, Mont- 
gomeryshire; common in the calcareous Bala schists and limestone of Cader Dinmael, near Corwen, Denbigh- 
shire ; in the fine Caradoc sandstone of Horderly W.; in the Bala schists of Rhiwargor, near Llanwddyn, 
Montgomeryshire ; very common in the Bala schists of Gelli Grin, Bala, Merionethshire; Bala schists of 
Cwm of the Cymmerig, Bala; in the lower Bala rock of Moel y Garnedd, Bala Lake; Bala schists of hill 
over Bala turnpike, N.E. of Bala Lake; Bala schists of Tan y Bwlch-y-Groes, S. of Bala, Merionethshire ; 
Bala limestone of Gaer Fawr, Welchpool, Montgomeryshire ; common in the Bala limestone of Coniston Water- 
Head, Lancashire; schists of Bwlch y Ciban; one doubtful specimen in the Bala schists near Corwen, Me- 
rionethshire; Bala schists of Moel Uchlas, Montgomeryshire; Bala schists of Llwyn-y-ci, N. W. of Bala; 
schists of Bala, Merionethshire; in the Upper Bala beds above Rother Bridge, Horton in Ribblesdale, York- 
shire ; Bala limestone, Llandeilo, Caermarthenshire; one doubtful specimen in the Bala schists of Pen Cerrig 
Serth, Builth; Bala schists of Wilfa, near Penmachno, N. Wales; a doubtful much depressed variety of 
the entering valve in which the dental lamelle seem to diverge at a less angle, and the rostral tooth 
much more slender, occurs with the ordinary types in the sandy Bala schists of Das Hithen ridge, Hirnant, 
Montgomeryshire, 
Genus. ORTHISINA (d@ Or.) 
Gen. Char.—External characters of Orthis, but the triangular pit in the cardinal area of the receiving 
valve closed by a concave or convex cicatrix, with an oval perforation near the apex (as in Strigocephalus); 
interior of receiving valve with two broad dental lamellee bordering the cardinal pit and converging to a mesial 
line at the surface of the shell; interior of entering valve with a trifid rostral tooth (as in Orthis), from which 
a small mesial septum extends towards the margin; lateral cardinal teeth as in Orthis. 
As the lines of growth are convex towards the beak and foramen on the deltidium-like inflexion of the 
shell, which as in Cyrtia fills the cardinal pit, the thought occurs that the foramen may be the result of disease ; 
but I can give no more than the suggestion from the specimens I have seen. 
ORTHISINA ASCENDENS (Pand. Sp.) 
Ref. and Syn.—Pronites adscendens Pander, Beitr. z. Geol. des Russ. Reiches, t. 17. f. 6. 
Sp. Ch.—Rotundato-quadrate, hinge-line slightly exceeding the width of the shell, in very small pointed 
ears; sides subparallel; front broad, often slightly sinuate in the middle, gently curved; receiving valve 
subpyramidal, greatest height at the point of the beak, from which the surface slopes almost straightly 
to the margins, the evenness of the surface interrupted by flat step-like zones of growth, about a line 
apart; cardinal area triangular, twice as wide as high, nearly vertical, or inclining towards the front 
margin, at an angle of 60°; pseudo-deltidium extremely wide, prominent, convex, crossed by nearly direct 
lines of growth, leaving a large irregular opening between its apex and the point of the beak; surface 
radiated with coarse, narrow ridges, irregularly bifurcating about once, separated by very deep sulci, narrower 
than the ridges (eight ridges in two lines at four lines from the beak). Width nine lines, proportional 
length of receiving valve =, depth of receiving valve ;j,: internal cast of receiving valve slit by a thick 
mesial septum, extending half the length of the valve, from which two dental lamellz diverge with a con- 
siderable curve to form the hood-shaped lateral boundaries of the cicatrized opening. 
I have not seen the entering valve of this species, but according to De Verneuil and Pander it is 
about half the depth of the receiving valve, gently convex in the middle, and with a nearly obsolete, linear, 
