BRITISH SILURIAN BRACHIOPODA. 157 



46. Rhynchonella. nucula, Sow., sp. Dav., Sil. Mon., PI. XXIV, figs. 1 — 7 ; Sup., 



PL X, figs. 27 to 29. 



At p. 181 of my * Silurian Monograph' I described Rh. nucula, and now again 

 refer to the subject in order, if possible, to point out the small differences by which the 

 shell under description and Bh. Llandoveriana are distinguishable. 



Ith. nucula is a very abundant fossil in the Upper Silurian, but varies considerably in 

 the number of its ribs ; these average from twelve to eighteen in different individuals, 

 while in Hit. Llandoveriana they are comparatively smaller, and number from eighteen to 

 thirty-six. In Bh. nucula usually four strong ribs occupy the fold, six to eight in Bh. 

 Llandoveriana. When the ribs are few in number, those on the lateral portions or lobes 

 of the shell are comparatively larger, and more nearly equal, than those that cover the 

 fold. 



As already stated, Bh. nucula abounds in the Ludlow, Wenlock, and Upper Llan- 

 dovery. The variety with few ribs, if we are correct in our identification, has been 

 found by Mrs. R. Gray in the Middle Caradoc at Shallock Mill, Girvan (' Sup.,' 

 PI. X, fig. 31), for we are unable to distinguish those Ayrshire specimens from others 

 that occur in the Upper Llandovery at Minsterley in Shropshire. Another variety (?) 

 (' Sup.,' PI. X, figs. 30, 30 «) has also been procured by the same indefatigable lady in 

 the Upper Llandeilo, or Balcletchie Conglomerate, at Balcletchie, Ayrshire ; at least, so 

 similar are they in shape to the typical forms of the species that we have not been able 

 to discover characters by which they might be distinguished, and I have therefore 

 thought it preferable to leave them provisionally with Bh. nucula than to give them a 

 separate name. 



47. Rhynchonella Llandoveriana, Bav. Sil. Mon., PI. XXIV, figs. 8—13 and 



14 (?) ; Sil. Sup., PI. X, figs. 32 to 37. 



We have already alluded to the differences which seem to distinguish Bh. Llan- 

 doveriana from Bh. nucula. We sometimes meet with elongated specimens in which the 

 posterior half of the shell tapers more than is usual in the larger number of individuals, 

 and this leads me to suspect that the so-called Bh. Weaveri, Salter, from the Upper 

 Llandovery is only an elongated form of Bh. Llandoveriana. In some specimens of the 

 last-named shell the ribs are very small and numerous. 



Bh. Llandoveriana was found by Mr. R. Philip in the Upper Llandovery at Speakes 



