ORTHID A, 219 
Obs. Dr. Lindstrém considers this shell to be distinguishable from O. elegantula by 
its general shape, the valves not converging or tapering anteriorly, as in the last-named 
species ; also by the deepish angular sinus which divides the dorsal valve into two dis- 
tinct parts or lobes. The cardinal and brachial processes are also smaller and slightly 
different in detail from those observable in O. elegantula. 
Position and Locality. In 1846 I found this shell in the Aymestry Limestone of 
Sedgley, near Wolverhampton, and Dr. Lindstrém assures me that my English specimens 
entirely agree with those that were found by himself in the Island of Gothland. 
ORTHIS POLYGRAMMA, Sow. (sp.), var. Penruanpica. Pl. XXIX, figs. 1—10. 
ATRYPA POLYGRAMMA, Sow. Sil. Syst., pl. xxi, fig. 4a, 1839. 
Orruis, sp., Salter. Memoirs of the Geol. Survey, 32, Scotland, p. 138, 1861. 
— Micueuini vel rEversa, Haswell. On the Silurian Formation of the Pentland 
Hills, p. 34, pl. iii, fig. 8, 1865. 
— poLyGRaMMaA, Murchison. Siluria, 3rd edit., p. 526, 1867. 
— —_ Dav. Trans. Geol. Soc. of Glasgow, Paleont. Series, vol. i, pl. ii, 
figs. 12 to 16, 1868. 
Spec. Char. Transversely oval, or about as wide as long, more often broadest anteriorly, 
front gently arched or slightly dented, sides moderately rounded, beak very obtusely 
angular ; hinge-line about half the breadth of the shell. Ventral valve convex, most so at 
the beak, while a wide shallow depression or sinus extends from the front to about one 
third of the length of the valve ; beak very slightly produced, incurved ; area triangular, 
narrow, fissure open. Dorsal valve rather deeper than the opposite one, uniformly convex, 
without fold. Surface of both valves marked by numerous thread-like radiating strie, 
augmenting in number, as the shell acquires age, by bifurcation or by numerous interpo- 
lated shorter strize. In the interior of the ventral valve a strong tooth is situated at each 
basal extremity of the fissure, while the curved dental plates enclose a large saucer-shaped 
muscular depression, longitudinally divided im the middle by a slightly raised mesial 
ridge. In the interior of the dorsal valve a small cardinal process is situated between 
two curved brachial processes, while the quadruple impressions of the adductor or 
occlusor muscle are longitudinally divided by a mesial convex ridge. ‘T'wo specimens 
measured— 
Length 11, width 13, depth 7 lines. 
Pemlioinss jes oy POLS Ss, 
Oés. This shell varies slightly in shape in different specimens, as may be seen by a 
glance at the forms illustrated in our plate. These modifications are chiefly due to some 
specimens being more regularly transversely oval than are others, and in the degree of 
convexity of the valves; thus, for example, in two individuals measuring about one inch 
