STROPHOMENID^. 321 



form, on the exterior of the ventral valve, undulating ridges (fig. 19 d). Its external 

 sculpture is very similar to what we find in Wahlenberg's type. 



We are not acquainted with the interior, which may perhaps present some corre- 

 sponding differences. It was found by Mrs. E. Gray, of Glasgow, in the Caradoc Lime- 

 stone of Craig-head Quarry, near Girvan, in Ayrshire. Dr. Gustav Lindstrom informs 

 me that a similar shell occurs in the Lower Silurian rocks of Sweden. I have seen six 

 or seven specimens of this shell, and they all agree in shape and character. 



Lept^na segmentum, Angelin. PI. XLVIII, figs. 28 — 30. 



LEPTiENA SEGMENTUM, .4«^e/m, MS. Musseum Palseontologicum Suecicum, 1838. 

 — — Lindstrom. Gotland's Brachiopoder, p. 374, 1860 ; and Revised 



List, 1867. 



Spec. Char. Shell small, semicircular, wider than long, broadest at the hinge-line ; 

 cardinal extremities prolonged into mucronate wings ; sides and front rounded. Ventral 

 valve moderately convex ; beak very small, scarcely projecting ; area moderately wide ; 

 fissure triangular, narrow, lanceolate, and entirely arched over by a small pseudo-deltidium. 

 Dorsal valve concave, following the curves of the opposite valve ; area narrow. Surface 

 of both valves bearing seven or eight principal, fine, thread-like radii, with interspaces 

 between each pair, very wide and longitudinally striated ; a short rib here and there 

 occupying the centre of the interspace in the vicinity of the margin. In the interior of 

 the ventral valve the muscular scars are very small, and scarcely defined ; a wide granulated 

 border surrounds the interior of the valve. In the interior of the dorsal valve the cardinal 

 process is minute, and situated between two small diverging brachial laminae ; the muscular 

 scars form two large oval depressions with raised margin, widely separated anteriorly. 

 Length 3, width 5, depth \ line. 



Ohs. This small shell has been for many years mistaken for the young of Leptana 

 transversalis. In 1838 it received from Prof. Angelin, who regarded it as distinct, the 

 specific designation of segmentum. In England, however, the name was not known prior 

 to Dr. Lindstrom's description of the species in 1860. No figure was appended; but 

 the shell was subsequently found in England by Dr. Lindstrom, and identified with the 

 Swedish type. L. segmentum differs from L. transversalis in many important particulars. 

 It is a much smaller species, considerably less convex, and more spindle-shaped ; its beak 

 scarcely projects. The area in both valves is comparatively wider, and the fissure and 

 pseudo-deltidium are much smaller and lanceolate, while the principal thread-like radii, 

 which ornament its valves, are considerably less in number, and have much wider 

 interspaces between them. In the interior of both valves the differences are likewise 

 great, as will be at once perceived by a glance at the figures of the two species given in 



41 



