BRITISH SILURIAN BRACHIOPODA. 171 



73. Lept^na Gray^, Dav. Sil. Sup., PI. XII, figs. 23—25. 



Shell marginally semi-circular, widest at the hinge-line ; ventral valve moderately 

 convex, and longitudinally keeled along the middle ; beak not projecting ; area triangular, 

 bent back at nearly straight angles to the plane of the valve ; fissure triangular, partly 

 arched over by a small deltidium ; dorsal valve moderately plano-concave, or flattened 

 laterally, and concave longitudinally along the middle ; hinge-area narrow ; surface of the 

 valves covered with thread-like radii, the interspaces between them being occupied by 

 ■one or two shorter radii. 



Length 7, width 11, depth 2 lines, 



Obs. — This is a well-marked species, differing from Leptoena transversalis and L. 

 sericea, and other forms of the kind, by the shape of its valves. The dorsal valve is 

 usually plano-concave, or is formed of three parts or lobes, that is to say, the lateral 

 ones are nearly flat, while the central lobe is concave, or, in other words, the median 

 depression commencing at the umbo gradually widens until it reaches the front, and 

 corresponds with the keeled part in the ventral one. The radii are often all of about 

 equal breadth. It is also a thicker shell than Lept. sericea. 



L. GvaycB occurs in the Upper Llandeilo at Craighead, Girvan. I name the species 

 after Mrs, Robert Gray, of Edinburgh, who has devoted so many years of her life to the 

 careful study and collecting of fossils in the Girvan district of Ayrshire. This species is 

 generically and specifically distinct from the small shell to which I gave the name of 

 Leptana Grayiim 1849, and which was subsequently discovered to be a synonym of 

 Sowerby's Chonetes {Lept.) minima. 



74. Lept^na Llandeiloensis, Dav. Sil. Sup., PI. XII, figs. 26 — 29. 



Shell marginally semi-circular; ventral valve evenly convex, area narrow, fissure 

 arched over with a small deltidium ; beak slightly incurved ; dorsal valve very gently 

 concave, sometimes almost flat ; hinge-line linear, surface of valves covered with thread- 

 like radii, with a shorter one interpolated between the longer pair. In the interior of the 

 dorsal valve, and at about a little more than half its length, there exists an elevated semi- 

 circular ridge, of which the margin is anteriorly much excavated and indented in the 

 middle, much scooped out posteriorly, and longitudinally divided into two large saucer- 

 shaped depressions by an elevated median ridge. On either side of the cardinal process 

 a prominent transverse ridge encircles the pits for the insertion of the articulating teeth 

 of the ventral valve ; under these and close on either side of the central ridge are situated 



