228 BRITISH PAL/AKOZOIC FOSSILS. [Bracuiopopa. 
receiving valve, with a triangular cardinal area about half the height of that in the receiving valve, but 
placed nearly in the plane of the lateral margins: both valves radiated with from fifty to sixty rather 
thick, flexuous, rounded ridges, frequently and irregularly branching between the beak and the margin; 
finer, closer, and less branched towards the sides ; about five in two lines in middle of front margin: internal 
east of receiving valve with two very long, strongly-marked, dental lamelle diverging at 40°, nearly straight, 
with a very slight inward curve at their extremities, forming the lateral boundaries of the very large tri- 
angular, prominent pair of muscular impressions, which reach rather more than two-thirds the length of the 
shell, finely suleated longitudinally, and separated by a slender, indistinct impression of a mesial ridge ; surface 
exterior to the muscular impressions strongly marked by the external ridges; internal cast of entering valve 
with a moderate elliptical pit of the simple rostral tooth, and two short, triangular, strongly-diverging impressions 
of the cardinal teeth ; immediately within and in front of which are the two obtuse obliquely conical bosses, 
produced by the muscular impressions, separated by an obtuse, short sulcus, from a rounded mesial ridge, not 
defined in front or at the sides; anterior pair of impressions invisible; surface sharply radiated with the 
external ridges. 
This species varies so much in form from compression in the schistose rocks, that I can scarcely venture to 
give the proportions, but I believe the true form to be pretty nearly such as I have figured in the above work, 
namely, about three times wider than long, with a straight front margin, and nearly rectangular sides ; but some 
specimens (distorted by pressure, in my opinion,) are as long as wide ; the average length is about seven lines. 
The species is very easily recognized by the coarse, frequently-branched, twig-like ridges in all the middle part 
of the shell, the lateral ones being more nearly straight, smaller, and frequently simple, and internally in the 
great length and distinctness of the prominent muscular impressions. The great number of the ridges 
separates it from the O. Actonia, to which however it has no real affinity in any of its internal or external 
characters. 
Position and Locality —Extremely common in the Bala schists of Llyn Ogwen, N. Wales. 
Explanation of Figures—P\. 1. H. fig. 25. Natural size of transversely elongated specimen, from the 
schists of Llyn Ogwen.—Fig. 26. Internal cast of receiving valve of possibly normal form, shewing the high 
cardinal area and long dental lamelle.—Fig. 27. Internal cast of entering valve, shewing the slender rostral 
and cardinal teeth.—Fig. 28. Longitudinal section of both valves. 
ORTHIS TESTUDINARIA (Dal.) 
Ref—Dal. Act. Holm. 1827, t. 2. f. 4. Sil. Syst. t. 20. f. 9 and 10. 
Sp. Ch.—Truncato-orbicular or rotundato-quadrate, depressed, receiving valve obtusely subcarinate along 
the middle, from whence the sides slope with very slight convexity to the margins; hinge-line almost as 
wide as the shell, the greatest width being slightly in front of it; the sides of the shell subparallel, front 
wide, with a very faint sinus in the margin towards the receiving valve: beaks small, pointed, not much 
incurved, but reaching the level of the lateral margins, from the backward inclination of the cardinal area, 
the height of which is only one-eighth of its width; entering valve slightly convex on the sides, with a 
narrow mesial hollow from the beak, becoming wide and hollow towards the margin; surface radiated 
with numerous strong, irregularly unequal, obtusely angular ridges, subalternate in height, separated by 
sulci about equalling them in width; the ribs are nearly simple and subequal for three or four lines from 
the beak, increasing in number towards the margin, both by intercalation of new ridges and by the 
branching of the old into three or four smaller, fasciculated by the greater strength of the original sulci, 
usually including three or five of the smaller towards the margin, the size of the ribs being greatest 
about the middle of the shell; at three lines from the beak of receiving valve about five to seven (most 
usually the former) strize in the space of two lines, about the same or slightly more numerous (about seven 
in same space), at the margin of adult specimens; surface finely granulo-punctate. Interior with sharply 
angular, narrow, extensions of the ribs nearly to the beak, separated by a wide space ; receiving valve with 
two very small diverging slits of cardinal teeth, from the ends of which a faint furrow encircles the muscular 
