BRITISH SILURIAN BRACHIOPODA. 181 



•98. Orthis sagittifbra, M'Coy. Dav., Sil. Mon., PI. XXXVI, figs. 18—23; SiL 



Sup., PI. XIII, fig. 29. 



A specimen closely resembling M'Coy's figure was procured by Mrs. R. Gray from 

 the Lower Llandovery at Mullock Hill, in Ayrshire. 



99. Orthis biforata, ScJdoth. Dav., Sil. Mon., PI. XXXVIII, figs. 11—25. 



This variable species is not rare in the Llandeilo at Craighead, in Ayrshire, and was 

 found by the Geological Survey of Scotland in the Caradoc at Duntercleugh Burn, Wan- 

 lock Water, Leadhills, Dumfriesshire. 



100. Orthis calligramma, Dav. Sil. Mon., PI. XXXV, figs. 1—24 ; PI. XXXVII, 



fig. 2; and Sil. Sup., PI. XIII, figs. 23—26. 



This very important and far spread Upper- and Lower- Silurian fossil has been fully 

 ■described at p. 240 of my ' Silurian Monograph.' It varies very much in shape as 

 well as in the number of its ribs. Its ribs are all straight and simple ; and in this 

 particular is distinguished from Orthis plicata, whose ribs are more numerous and with 

 shorter ones interpolated between the longer pair. If Mr. Salter is correct in con- 

 sidering the 0. plicata as a variety of 0. calligramma, the distinction between it and 

 Dalman's species would be unfounded. Under the prevailing uncertainty it is, I think, 

 preferable to maintain 0. calligramma and 0. plicata as distinct species. 



In Scotland, undoubted examples of 0. calligramma have been found by Mrs. R. 

 Gray in the Llandeilo at Craighead, Ardmillan Brae, Balcletchie, in the Girvan 

 district. In the Middle Caradoc, at Shallock Mill ; at Thraive in the Upper Caradoc; at 

 Shallock Porge, in the Upper Llandovery ; and at Mulloch Hill, in the Lower Llandovery 

 of the same district. Again, it occurs in vast numbers and of large size in the Penkill 

 Limestone at Penwhapple Glen, Girvan. It has been collected by the Geological Survey 

 •of Scotland in the Caradoc at Glencotho Quarry, near Broughton, in Peebleshire ; at 

 Duntercleugh Burn, Wanlock Water, and Black Burn, Leadhills District, Dumfriesshire; 

 also in same formation at Snar Water, in Lanarkshire ; and under the name of 0. 

 Davidsoni is found in the Wenlock Limestone, near Walsall, in Staffordshire, so that its 

 vertical range extends from the Llandeilo up to the Wenlock inclusive. The var. 

 ^cotica is only a form of calligramma. 



