BRITISH SILURIAN BRACHIOPODA. 177 



convex, deeper than the dorsal one, and longitudinally depressed along the middle ; beak 

 obtusely angular, moderately incurved, and bent back at an almost obtuse angle to the 

 plane of the valve ; area large, triangular, fissure open ; dorsal valve moderately convex, 

 longitudinally depressed along the middle, cardinal extremities acutely angular. Surface 

 of valve ornamented with a variable number of small radiating ribs with shorter ones 

 interpolated between the principal pairs. Surface of valves crossed with equidistant, 

 prominent, scale-like, concentric lines or ridges. 

 Length 6, breadth 7, depth 4j lines. 



Obs. — For a long time I felt uncertain whether this Llandeilo fossil was more 

 than a variety of the Wenlock OrtJds Bouchardii, but after much comparison I find it 

 presents peculiarities which, added to the great difierence in stratigraphical position, 

 make it desirable to obtain for it a distinct specific name. It is closely allied to Orthis 

 Bouchardii, but is slightly larger and possesses a greater number of ribs. The mesial 

 depressions in both valves is less deep than in 0. Bouchardii. 



At p. 210 of my 'Silurian Monograph' it is erroneously referred to the Lower- 

 Llandovery or Caradoc of Penwhapple Glen, near Girvan. It was subsequently ascer- 

 tained by Mrs. R. Gray that its true position is the Llandeilo at Craighead and Balcletchie, 

 in Ayrshire. 



85. Orthis nina, Dav. Sil. Sup., PL XIII, fig. 11. 



Shell minute, semi-circular, or obscurely subpentagonal, wider than long; dorsal 

 valve moderately convex, longitudinally divided by a somewhat angular or roof-shaped 

 mesial fold ; ventral valve convex and divided longitudinally by a moderately deep and 

 somewhat angular sinus ; beak gently incurved, area triangular, moderately large and 

 divided by an open fissure. Surface of valves marked with numerous narrow, thread-like 

 radii and wide interspaces, along the middle of which is a shorter riblet. 

 Length 1, breadth 1^ lines. 



Obs. — This minute species of Orthis seems abundant in the Middle Caradoc at 

 Whitehouse Bay, near Girvan, but Mrs. R. Gray states it to be rarely found in good 

 preservation. 



86. Orthis unguis. Sow. Sil. Mon., PI. XXXVII, figs. 16—22 ; Sil. Sup., PI. XIII, 

 fig. 28. 



I am very uncertain whether this species really occurs in Scotland. I have figured in 

 Plate XIII of this Supplement a specimen from the Llandeilo of Craighead, which 

 seems identical with some typical examples from Gretton in England. The identification 

 will, however, lequire to be confirmed by the discovery of better examples. 



