180 SUPPLEMENT TO THE 



94. Orthis biloba, LinnL Dav., Sil. Mon., PI. XXVI, figs. 10 — 15. 



This well-known shell is abundant in the Pentland Hills, Ludlow Rocks, but 

 is apparently very rare in Scotland in lower beds. Mrs. R. Gray found two examples of 

 the fossil in the Middle Caradoc at Shallock Mill, in Ayrshire. 



95. Orthis confinis, Salter. Dav., Sil. Mon., PL XXXVI, figs. 1—4. 



This species is fully described and illustrated at p. 266 of my ' Sil. Mon.' It is a 

 most abundant fossil in the Llandeilo of Scotland, and besides at the place assigned 

 to it by Mr. Salter it has been found by Mrs. P. Gray at Doularg, Ardmillan Brae, 

 Craighead, Minuntion, all in the Llandeilo of Ayrshire; bivalve specimens are not, 

 however, very abundant, the shell being usually found in separate valves. I am not 

 certain whether the fossil has been found either in the Caradoc or Llandovery, although 

 some specimens from the Lower Llandovery of Mullock Hill have been attributed to it. 



96. Orthis reversa, Salter. Sil. Mon., PI. XXIX, figs. 11—13; Sil. Sup., PI. XVI, 



fig. 13. 



A specimen of this species was found by Mrs. R. Gray in the Upper Llandovery, or 

 Penkill Beds, at Penwhapple Glen, Girvan. 



97. Orthis Mullockiensis, Dav. Sil. Mon., PI. XXIX, figs. 14—18; Sil. Sup., PI. 



XVI, figs. 14, 15. 



At p. 221 of my ' Silurian Monograph ' this fossil has been described as a variety of 

 Orthis reversa, Salter, but it may be specifically distinct on account of the marked sinus and 

 fold in its valves. In Scotland it is an extremely abundant fossil in the Lower Llandovery 

 at Mullock Hill, three miles north-north-east of Girvan, and it was found by ]\Irs. R. 

 Gray in the Upper Caradoc at Thraive, in the same district of Ayrshire. 



