1G8 SUPPLEMENT TO THE 



G6. LEPTiENA YouNGiANA, Dav. Sil. Mon., PI. XLVII, figs. 19, 20 ; Sil. Sup., PI. XII, 



figs. 14—16. 



At p. 320 of my ' Silurian Monograph,' this shell is described as a variety of Lepf. 

 transversalis. Since then I have been able to examine a large series of fine examples of 

 its exterior and interior, and am disposed to consider it specifically separable from the 

 Upper-Silurian Z. transversalis. It varies very much in shape as may be seen from the 

 figures. Two fully-developed examples from the Upper Llandeilo of Craighead, in the 

 collection of Mrs. R. Gray, measure — one, length 12, breadth 12 lines; the other, length 

 12, breadth 9 lines. 



The one being almost circular, the other elongated oval. The ventral valve is very 

 convex, beak large and much incurved, area narrow ; dorsal valve concave, following the 

 curves of the opposite one. Hinge-line less than the breadth of the shell. The larger 

 number of specimens are nearly circular and about as broad as long ; the radii and striae 

 that cover the surface of the valves are also exceedingly fine and often hardly visible. 



67. Lept^na tenuicincta, M'Coy. Sil. Mon., PI. XLVII, figs. 7—18; Sup., PI. 



XII, figs. 17—21. 



I have nothing to add to the description I have given of this well-marked species at 

 p. 326 of the ' Silurian Monograph.' It is, however, an extremely variable shell as to 

 length and breadth. I have in my possession two internal casts from the Caradoc of 

 Cynwyd, near Corwen, in Merionethshire, measuring, respectively, length 13, breadth 6 

 lines, and length 9, breadth 12 hnes, and every possible intermediate dimension and 

 relative proportion as to length and breadth may be collected. In Scotland it has been 

 found by Mrs. R. Gray in the Caradoc at Sballock Mill, in Ayrshire, and by the Geological 

 Survey of Scotland in rocks of a similar age at Wrae and Kilbucho (near the church), two 

 and a quarter miles west of Culter, in Peebleshire. At Dunterleugh Burn, Wanlock 

 Waters, Leadhills District in Dumfriesshire, and at Glentrip Burn, Wanlock, in 

 Lanarkshire. In the Upper Llandeilo it is abundant at Ardmillan Brae, Craighead, and 

 Balcletchie, in the Girvan district, Ayrshire. The English and Irish localities have been 

 already recorded. 



