182 SUPPLEMENT TO THE 



101. Orthis PLicATA, Sow. Sil. Moil., PL XXXV, figs. 25, 26 j and PL XXXVII,. 

 figs. 1,2; SiL Sup., PL XIII, fig. 27. 



This species has been collected by ]\Irs. R. Gray from the Middle Llandovery at 

 Woodland Point and from the Llandeilo at Ardmillan Brae, in Ayrshire. It occurs like- 

 wise in the Caradoc. 



102. Orthis Carausii, Salter. Dav., Sil. Mon., PL XXXIII, figs. 1—7 ; and SiL Sup., 



PL XIV, figs. 21—26. 



Orthis calligeamma, var., Salter. Appendix to Eamsay's Memoir on the Geology 



of North Wales, p. 258, pi. xxii, fig. 1, 1866. 



— Carausii, Salter, MS. Davidson, Geol. Mag., vol. v, p. 315, pi. xvi, fig. 23, 18C8. 



— — , Salter. A Catalogue of the Collection of the Camhrian and Silurian 



Fossils in the Museum of the University of Cambridge, 

 p. 24, 1873. 



At p. 229 of my ' Silurian Monograph' I described and figured a series of specimens 

 to which Mr. Salter had given the MS. names of 0. Carausii and 0. Hicksii. Since 

 then it has been suggested by Prof. T. McKenny Hughes that the two above-named 

 fossils were one and the same species. In his first paper on the " Geology of Anglesey," 

 'Quart. Journ. Geol. Soc.,' vol. xxxvi, p. 238, 1880, Professor Hughes refers to 0. 

 Carausii as a characteristic Tremadoc Eossil, and adds, in his second paper on the 

 geology of the same locality, ' Quart. Journ. Geol. Soc.,' vol. xxxviii, p. 26, 1882, that 

 " these species both belong to the 0. Actonia group, differing from the 0. calligramma 

 group in the relative shallowness of the dorsal valve, being almost plano-convex, and from 

 O.flaheUulumiy^Q in having the ventral valve the more convex. It will be observed from 

 the descriptions in Davidson that the two species agree in every particidar till we come 

 to the absolute size, the relative length and width, and the number and arrangement of 

 the ribs. Prom the specimens exhibited it will be seen that the relation of length to 

 width is not constant, that the size is a question of age, and also that in respect to the 

 number and arrangement of ribs, every intermediate form between 0. Hicksii and 0. 

 Carausii is represented in the specimens procured from one and the same bed in Anglesey ; 

 therefore we must put this down as an early form of Orfhis following the receding shore 

 in the Cambrian times through the long ages that elapsed between the deposition of the 

 rocks known as Menevian and that of the beds called Arenig." I much regret that I 

 cannot coincide with Prof. Hughes in the views he has expressed relative to the two 

 species under discussion. 



