160 SUPPLEMENT TO THE 



52. RiiYNCHONELLA Balcletchiensis, Bav. Sil. Sup., PI. X, figs. 15, 16. 



Shell sub-pentagonal, about as wide as long, broadest anteriorly, tapering posteriorly ; 

 dorsal valve regularly convex to about half its length, fold very wide, large, sub-angular, 

 with a short, shallow, longitudinal groove along its middle ; lateral slopes of fold large; 

 anterior portions of the valve convex, forming a short, rounded rib at the base of the 

 fold ; ventral valve most convex near the beak. At about half its length commences a 

 large, deep sinus, divided in the middle by a short, rounded rib ; beak incurved. 

 Length 5, width 5, depth 3 lines. 

 Ohs. — This is a very remarkable species with a wide biplicated fold and a large 

 triplicated sinus ; the two lateral ribs forming the margin of the sinus commence at 

 about one third of the length of the valve and then deviate. The mesial rib is small 



Ith. Balcletchiensis was found by Mrs. R. Gray in the Upper Llandeilo, at Craighead, 

 and at Balcletchie, Girvan, Ayrshire. 



53. Rhynchonella gemula, Salter, MS. Dav., Sil. Mon., PL XXIV, fig. 21 ; and Sil. 



Sup., PI. X, fig. 17? 



Of this remarkable species several good examples were found by Mrs. Robert Gray 

 in the Upper Llandeilo at Craighead. In my description of the species at p. 188 of the 

 ' Silurian Monograph,' it is quoted from the ' Caradoc ' of Craighead, but subsequently 

 the limestone in that locality was attributed by Prof. Lapworth to the Upper Llandeilo. 

 All the species in my ' Silurian Monograph,' quoted from Craighead, will consequently 

 be of Upper Llandeilo age and not Caradoc. 



54. Rhynchonella nasuta, M'Coy. Dav., Sil. Mon., PI. XXIII, fig. 19 ; Sil. Sup., 



PI. X, figs. 20, 21 ? 



Since describing this species at p. 173 of my ' Silurian Monograph,' I have been able 

 to examine a number of specimens of the shell collected by Mrs. R. Gray from the Upper 

 Llandeilo at Craighead and Balcletchie. These examples show that the shell was not 

 always elongate as described and figured by jM'Coy ; the larger number of specimens 

 are not much longer than wide. It is evidently a very variable shell, some twenty-six 

 ribs may be usually counted on each valve, but as many as thirty-two are sometimes 

 observable. The ribs on the lateral lobes of the shell are smaller than those that occupy 

 the fold and sinus. 



