BRITISH SILURIAN BRACHIOPODA. 175 



and description is necessarily incomplete. Knowing nothing of the interior of the dorsal 

 valve, I cannot say whether it possessed the internal character of the genus to which 

 it is provisionally referred. It bears, however, much resemblance to other species of 

 Skenidium. The shell was found by Mrs. R. Gray in the Middle Caradoc at Shallock 

 Mill, Girvan. 



80. Skenidium GRAYiiE, Bav. Sil. Sup., PI. XI, figs. 3, 4, 5. 



Shell nearly circular and about as wide as long, ventral valve conical, uniformly 

 convex without sinus, beak nearly straight and bent back at a very obtuse angle to the 

 plane of the valve. Area large, fissure triangular ; surface of valves covered with 

 numerous fine riblets, shorter ones being interpolated between each principal pair. In 

 the interior of the ventral valve a mesial septum supports a comparatively large saucer- 

 shaped dental-plate, concave, and divided into two portions by a narrow ridge ; dorsal 

 valve not known. 



Length 7 hues by about the same in breadth, and 4i lines in depth, 



Obs. — Of this species Mrs. R. Gray found an impression of the ventral valve, from 

 wliicli the exterior of the valve has been reproduced by the means of gutta-percha ; also an 

 internal cast of the same valve, which also I moulded in gutta-percha. These specimens 

 were procured from the Middle Caradoc at Thraive, Girvan, Ayrshire. If my generic 

 identification proves to be correct, this is the largest species of the genus with which we 

 are at present acquainted. Until the dorsal valve has been discovered the description and 

 identification must be considered as incomplete and provisional. 



Genus — Orthisina, d'Orbigny. 1849. 



81. Orthisina adscendens, Pander, sp. Dav., Sil. Mon., PI. XLIX, figs. 27 — 29 ; 



Sup., PI. XVI, figs. 16, 17, 18. 



Since the describing and figuring of this species at p. 278 of my ' Silurian] Mono- 

 graph,' better specimens have been collected by my zealous friend, Mr. John Parrott, from 

 the Upper Caradoc or Bwlch-y-gassi Beds, near Cynwd ; and these have enabled me to 

 add figures of the external and internal surfaces of the dorsal valve. This valve is either 

 nearly flat or very slightly convex, with a feebly marked longitudinal depression. The shell 

 itself had not been hitherto found in our British rocks, but excellent impressions and 

 casts of both valves are not uncommon, so that by the means of white gutta-percha the 

 exterior and interior of both valves can be easily reproduced. 



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