210 BRITISH SILURIAN BRACHIOPODA. 
broadest anteriorly. Hinge-line shorter than the width of shell; valves convex, becoming 
attenuated at the margin. Dorsal valve moderately convex, slightly gibbous near the 
umbo, with a mesial depression extending to the front ; hinge-area narrow. Ventral valve 
slightly deeper than the opposite one, somewhat depressed longitudinally along the middle; 
beak produced, acutely angular, very slightly incurved; area large, triangular; fissure 
linguate, open. Surface of each valve marked by a variable number of strong obtusely 
angular ribs, which increase in number with age, one or two shorter and smaller ribs 
becoming interpolated between each pair of larger ones, while sometimes some of the 
principal ribs become bifurcated. The surface of each valve is also strongly imbricated 
or crossed by prominent, equidistant, scale-like, concentric ridges. In the interior of the 
dorsal valve a small prominent cardinal process is situated between two curved brachial 
processes. Under the cardinal process commences a prominent convex ridge, which 
separates each pair of the quadruple muscular scars. In the interior of the ventral valve 
a prominent tooth is situated at each extremity of the base of fissure, while the dental 
plates curve inwards and enclose a saucer-shaped muscular cavity, divided along the 
middle by a widish, slightly raised mesial ridge, which is itself longitudinally depressed - 
along its middle. Shell-structure punctate. 
Length 63, width 7, depth 4 lines. 
Obs. This is a rather small but very distinct and interesting little species, well 
characterised by the arrangement of the coste and their strongly imbricated surface. It 
is also a very thick shell. As will be seen by a glance at the figures in our plate, the 
number of ribs increase rapidly with age, and show much regularity or symmetry in their 
arrangement in the same specimen. When quite young, as in fig. 16, six or eight simple 
ribs only are present on each valve; in fig. 17 they become more numerous, chiefly from 
interpolations, and still more so in the adult examples, fig. 18. Although the shell is 
generally smaller than that of the proportions above given, some few specimens have 
slightly exceeded them. 
Position and Locality. The true geological position of this species is the Wenlock 
Limestone, and in which formation at Benthall Edge I found the first example in 1846. 
In ‘Siluria’ it is quoted from the Llandovery rocks, while the Museum of Practical 
Geology of London possesses a well-preserved ventral valve (fig. 23), which is stated to 
have been found in the Lower Llandovery or the Caradoc at Penwhapple Glen, near Girvan, 
in Ayrshire. 
Abroad it has been collected by Dr. Lindstrém in the Upper Silurian rocks of the Island 
of Gothland. 
