ORTHID A. 209 
spaces of about equal width. ‘The ribs are either simple, bifurcated, or due to the inter- 
polation of a shorter rib between each two of the longer ones. In the interior of the dorsal 
valve an elevated, thin, triangular, mesial septum extends from under the extremity of the 
umbonal beak to within a short distance of the front. The two curved brachial processes 
are connected with the upper edge of the septum by the means of two concave plates ; on 
the outer side of the brachial processes, close to the hinge-line, are situated the sockets, 
while a pair of muscular scars occupy the bottom of the valve on either side close to the 
mesial septum. In the interior of the ventral valve a prominent tooth is situated at the 
base of the fissure; while the dental plates converge on either side, and enclose a small 
saucer-shaped cavity, longitudinally divided by a mesial ridge. 
Length 33, width 33, depth 3 lines. 
Oés. This pretty little Orthis differs much from its congeners by the internal arrange- 
ments of its smaller valve, and is the only British species with which I am acquainted 
presenting in the interior of its dorsal valve an elevated triangular septum with two con- 
cave plates connecting the brachial processes with the upper edge of the septum. In the 
Devonian rocks of Ferques, in France, we find another small Orthis (O. Deshayesii, Bou- 
chard, MS.) possessing a similarly shaped septum, but without the curved plates which in 
O. Lewisii connect the brachial processes with the mesial septum. In O. Lewisii there does 
not appear to exist any prominent cardinal process, ‘Ihe interior of the ventral valve 
does not, however, appear to differ in any marked peculiarity from what we find in the 
generality of species of which the genus is composed. 
Position and Locality. O. Lewisii occurs in the Wenlock Limestone at Dudley and at 
Hay Head, near Walsall. 
In Scotland it is found in beds of a similar age in the Pentland Hills. In this last- 
named locality well-preserved internal casts are often procured. 
Abroad it was found by Dr. Lindstrom in the Island of Gothland. 
Ortuis Boucnarpi, Dav. Pl. XXVI, figs. 16—23. 
Ortuis Boucnarpil, Dav. London Geol. Journal, p. 64, pl. xiii, figs. 5—8, Feb., 
1847. 
— — Dav. Bull. Soc. Géol. de France, 2nd ser., vol. v, p. 322, pl. iii, 
fig. 20, 1848. 
— — Morris. Catalogue of British Fossils, p. 139, 1854. 
— — Salter. Siluria, 2nd ed., p. 251, fig. 1, 1859. 
— — Lindstrém. Gotlands Brachiopoder, p. 371, 1860. 
Spec. Char. Somewhat pentagonal or quadrate, thick, rather wider than long, straight 
or slightly indented in front, tapering at the beak, moderately rounded or arched laterally; 
27 
