172 SUPPLEMENT TO THE 



the muscular depressions or scars. In the interior of the ventral valve there exists like- 

 Avise, at about half its length, a raised, semicircular ridge, similar to the one in the dorsal 

 valve, but not indented in front nor divided into two parts, but forming a regular concave 

 space between its raised margin and the hinge-line. On either side of the fissure are the 

 articulating teeth, with a four-lobed muscular space with raised angular margins. The 

 interior surface of both valves is marked with finely-indented radiating lines. 

 Length 9, breadth 13 lines. 



Obs. — In 1867 my attention was called by Mr. J. Young, of Glasgow, to a fine 

 interior of the shell under description. For a long time I felt uncertain as to the genus 

 to which it should be referred. Subsequently four more examples were handed to me 

 by Mrs. R. Gray, and I am now disposed to consider them as probably belonging to a 

 new species of Leptcena. These were obtained by Mrs. R. Gray from the Upper Llandeilo 

 at Craighead and Ardmillan Brae in Ayrshire. 



In the Explanation of Sheet 3 of the Geological Map of Scotland this species is referred 

 to Leptcena scissa, but in scissa we do not find the very remarkable and characteristic 

 elevated ridges peculiar to Leptcena Llandeiloensis, and I now question whether any true 

 L. scissa has been discovered as low down as the Llandeilo. Lep. scissa is a Llandovery 

 form, but is said to have been found likewise in the Caradoc in Ireland, but this will require 

 further confirmation. One small internal cast, showing the interior of both valves, and 

 which I take to be a young Z. Llandeiloensis from Ardmillan Brae, has enabled me to 

 determine the characters of the ventral valve. 



Genus — Streptorhynchus, Kin^, 1850. 



75. STRErTORHYNCHUs NASUTUs, Lindsfrom. Dav., Sil. Mon., PI. XXV, figs. 1 and 2. 



Strophomena nasuta, Lindstrom. 1860. 

 Cyktia? nasuta, Dav. Sil. Mon., p. 201, 1869. 



Streptoehynchus nasutus, Lindstrom. Fragmenta Silurica, p. 28, pi. xvii, figs. 7 



—10, and tab. xix, figs. 17, 18, 1S80. 



For a long time past much uncertainty has prevailed respecting the genus to which 

 this small species should be referred. In my 'Silurian Monograph' (p. 201) I 

 doubtfully referred it to Cyrtia on account of its very spirifer-like appearance. Since then 

 Professor Lindstrom has been so fortunate as to procure interiors of the shell, which 

 show that its proper place is with Streptorhynchis. 



This curious little species is apparently very rare n our British Silurian Rocks, for 



