242 BRITISH PALAZOZOIC FOSSILS. [ Bracutopopa. 
the radiating ridges, which are very prominent, irregular in size, from abrupt irregular swellings and dimi- 
nutions of thickness in various parts of their length; irregularly branching three or four times so as to 
retain their average thickness throughout; seven or eight in two lines about the middle of the shell, about 
five in the same space at the margin at ten lines from the beak; very rugged, from close, strong, scaly 
lines of growth. Average width ten lines, proportional length *,, depth =. 
The above quoted figure very imperfectly represents the suleation of this species. 
Position and Locality—Common in the Coniston (Bala) limestone of Coniston Water-Head, Lancashire ; 
common in the Bala slates of Blaen y Cwm, Nantyre, Glyn Ceiriog, S. of Llangollen, Denbighshire ; com- 
mon in the Bala slates of Llansantfraid, Glyn Ceiriog, Denbighshire; Bala limestone of Mathyrafal, S. of 
Meifod, Montgomeryshire. 
Lerrana (Strophomena) COMPRESSA (Soi. Sp.) 
Ref. and Syn. = Orthis compressa Sow. Sil. Syst. t. 22. f. 12. 
Sp. Ch.—Transversely oblong, subquadrate ; hinge-line slightly exceeding the width of the shell, sometimes 
forming short acute ears, from a slight outward curve of the margins near the angles, in other specimens 
nearly rectangular; lateral margins slightly convex, subparallel, front margins slightly convex, nearly as wide 
as the shell; valves much depressed, and concentrically marked with two or three strong undulations of growth 
from one to two lines apart; receiving valve slightly convex, with a low triangular cardinal area, inclined 
backwards at about 120°; with a wide triangular foramen, apparently more than half closed by a pseudo- 
deltidium: entering valve very slightly convex near the beak, flattened in the middle, and very slightly 
concave towards the margin, with a linear, nearly obsolete cardinal area or line: strixe of both valves slightly 
arched divaricatingly on the sides, close, obtuse, transversely striated, nearly equal or subalternate, increasing 
in number, but not much in size, towards the margin; eighteen in the space of two lines at six lines from 
the beak; the separating sulci narrower than the ridges, and regularly pitted with very large punctures, 
visible through all the thickness of the shell, to the internal cast, and so large as slightly to undulate 
the radiating strie; in some specimens faint traces of larger strie at every third or fifth line traceable : 
internal cast of entering valve nearly smooth, with faint punctured radiating lines, visible with a lens (cor- 
responding with the external strie); one long elliptical pit produced by the simple rostral tooth, and two 
ovate pits of the thick cardinal teeth, only equalling the rostral tooth in length, and diverging at 95°; between 
them and the hinge-line a small elevation on each side for the cardinal pits; muscular impressions very 
small, reaching little more than two-fifths the length of the shell, subrhomboidal, obscurely divided by a 
few obtuse sulci on their anterior edges, separated by the broad sulcus of a flattened mesial ridge, not 
reaching quite their length; receiving valve with two short thick dental lamells diverging at about 130°; 
muscular impressions very broad, faintly radiated with fine sulci, indistinctly defined except by the straight 
diverging dental lamelle, divided by a long slender sulcus of a linear mesial ridge, reaching with the 
impressions about half the length of the shell. Width one inch, comparative length =, depth ji. 
The great compression of the valves, thinness of the shell, the marked concentric waves of growth, thread- 
like, subequal strize, and large punctures, easily distinguish this, in well-marked specimens, from the O. expansa 
or O. alternata, as well as the difference in the cardinal area and foramen and muscular impressions, and simple 
rostral tooth. I have little doubt the shell figured by Sowerby as the O. anomala (Schlot.) Sil. Syst. t. 21. f. 10, 
is a very old specimen of this species, elongated by a compression, which has also inereased its convexity. Such 
specimens are not uncommon, indicating apparently the thickened adult state, in which it often shews an approach 
to S. grandis, when, as sometimes happens, some of the striz in part of a specimen become more pro- 
minent than the others. 
Position and Locality—Very common in the fine Caradoc sandstone of Horderly W.; in the Bala lime- 
stone of Teirw river, S. of Llangollen, N. Wales; extremely common in the calcareous Bala schists and 
limestone of Craig y glyn, N. of Rhaider, near Llanarmonfach, E. of the Berwyns ; Bala schists of Moel Uchlas, 
Montgomeryshire ; Bala schists of Bryn Eithin, Penmachno, N. Wales; Bala schists of Cader Dinmael, near 
Corwen, Denbighshire; Bala ?limestone of Tyrnant, H. of Llanwddyn; Bala schists of the Cymmerig, E. of 
