BRITISH SILURIAN BRACHIOPODA. 167 



On the surface of both valves from four to six narrow, thread-Uke rays radiate at about 

 equal distances, and between them the surface is very finely longitudinally striated . 

 dorsal valve concave, divided into three parts by divaricating grooves corresponding to the 

 two large ribs in the ventral valve ; hinge-line narrow ; interior not known. 

 Length 2, breadth nearly 6 lines, 



Ois. — While sorting some specimens of Leptcena segmentum obtained by Mr. G. 

 Maw from the washings of upwards of a ton weight of Middle- Wenlock Shale from 

 Coalbrook-Dale Station, and half a mile from Buildwas Abbey in Shropshire, my attention 

 was drawn to several very remarkable shells which differed so much from the great bulk of 

 specimens of L. segmentum derived from the Buildwas or Lower- Wenlock Shales, that I 

 felt uncertain whether these abnormal shells might not be malformations of Angelin's L. 

 segmentum ; but, as the same character was regularly reproduced on several of the speci- 

 mens, I considered it desirable to give them a separate varietal designation. 



Leptana segmentum, Angelin, is a small, regularly semi-circular shell, rarely exceeding 

 3 lines in length by 6 in breadth ; its ventral valve uniformly convex and marked by 

 seven or eight principal, thread-like radii, with interspaces finely longitudinally striated. 

 Between the principal radii there is often a shorter riblet, and very rarely, but occasionally, 

 a tendency in some specimens to shght marginal undulation, and to the formation of two 

 or three larger rounded ribs, but to a much smaller extent than in the specimens of the 

 variety cornuta figured in my plate. I believe that the extreme variation observable 

 between L. segmentum proper and the variety cornuta may be connected by intermediate 

 or passage forms. 



Mr. Maw's washings placed within my hands some 2000 specimens of Leptcena 

 transversalis and Lept. segmentum, and I found the two forms always easy to separate. 

 Lept. transversalis is a much larger species than L. segmentum, and its length is greater 

 in proportion to its breadth than in Angelin's shell ; the beak also of L, segmentum is 

 very small and hardly visible beyond the area, whilst in L. transversalis it is comparatively 

 larger and much incurved. The thread-like radii are likewise much more numerous and 

 close in Dalman's than in Angelin's species. 



Leptcena segmentum was found by Mrs. R. Gray at Cuddyston Glen in the Upper 

 Llandovery, and at Woodland Point in the Middle Llandovery, both in the Girvan 

 district of Ayrshire. Lept. transversalis was also found by Mrs. R. Gray in the Upper 

 Llandovery at Penkill, and in the Middle Llandovery at Woodland Point, also in Ayr- 

 shire. The shell occurs in the Ludlow Shales of the Pentland Hills. 



23 



