356 BRACHIOPODA OE THE 



lation of the description Rouault gave of his shell. " It is a large shell of the arcuato- 

 striata group of M. de Verneuil, not very thick ; its dorsal valve moderately convex, with 



a slightly projecting beak ; both valves are covered with 



^^jgff'^jjw very numerous, fine, radiating striae and well-marked 



/0a ';b. concentric lines of growth. The most prominent charac- 



Jj|||l|> ; ' ^ik ter > however, and at the same time the most difficult to 



A ^Ifi define in a specific description, is furnished by the nume- 



fB§Pf§-; rous deformations it has undergone; these deformations 



i&jjm, to 1 are such that, out of more than sixty specimens, it has been 



^w-; -W impossible to find two alike, although they all belong to 



^"■^iUMiLuJ^^ a species of which the symmetrical form is well recog- 



OrfM.ArftoM.tjpe. nised „ M Rouault > s type examp i e t he one figured 



M. Rouault's figure. Slates of La J r r ° 



Couyere. above, measures — 



Length 19, breadth 19 lines. 

 Of this species I have seen a number of specimens from La Couyere, the same locality 

 from whence M. Rouault obtained his types. They all occur in black slates, and 

 are flattened by compression. Some show the exterior ; others are internal casts, on 

 which the muscular scars are visible. These slates occur above the " Gres Armoricain " 

 and under the " Gres de May." As far as my knowledge goes, we have not found in 

 England slates with specimens of this large Orthis. 



38. Orthis Berthoisi, var. erratica, Bav. Sil. Mon., p. 233, PI. XXXII, figs. 21 



—28 ; B. S. Sup., PI. XLI, figs. 1—9. 



This shell has been fully described and illustrated in my Silurian Monograph j but, as 

 some better examples have turned up, it has been considered desirable to add figures of 

 a few of them to this Supplement, as well as to give some further details with respect to 

 its character and history. 



As has been already stated at p. 234 of my Monograph, Mr. D. Sharpe, in 1853, 

 described and figured an Orthis, from the Lower-Silurian Rocks of Portugal, under the 

 name of 0. Berthoisi, Rouault. It is very variable in shape, as may be seen from the 

 good series of Portuguese specimens in the possession of the Geological Society of London ; 

 but it is a much smaller shell than Rouault's typical form from the slates of La Couyere, 

 not being more than half its size ; and in this respect it agrees completely with the 

 specimens that are found in the quartzite pebbles of Budleigh-Salterton. In the gritty, 

 hard sandstones of St. Germain-sur-Ille, and of La Bouexiere, as well as in the reddish 

 quartzites of May, in Normandy, there occur vast numbers of a shell exactly similar to 

 those that are found in Portugal and at Budleigh-Salterton. This rock is higher up in the 



