224. BRITISH SILURIAN BRACHIOPODA. 
projections and indentations produce corresponding depressions and elevations on the 
internal casts of both valves.) In the interior of the ventral valve the muscular area is 
rather large, and margined by a slightly projecting ridge, while a widish ridge extends 
longitudinally along the centre. 
Length 11, width 12 lines. 
Obs. I do not feel certain that the shell above described is really identical with the 
Orthis Berthotsi of Rouault, and consequently have given to it the varietal designation of 
erratica, which can be hereafter retained as a specific denomination should a comparison 
with Rouault’s type prove the two to be distinct. The shell figured by the French 
paleontologist assumes much larger proportions than any specimen I have yet seen of the 
shell under description from Budleigh-Salterton. The specimen of O. Berthois: found by 
M. Rouault at Couyére, in Brittany, were much distorted from the effects of cleavage; but 
one example, with apparently its natural shape, measured about 19 lines in length by 
about the same in breadth, while the hinge-line and area were also smaller than in any of 
the British examples provisionally referred to it.* 
In 1853 My. D. Sharpe figured and described under the designation of O. Berthoisi, 
Rouault, a shell much more resembling our Budleigh-Salterton examples, and which he 
stated to be abundant in the upper division of the Lower Silurian formation at the Porto 
di Santa Anna, in the Serra de Bussaco, in Portugal. Mr. Sharpe observes’ that “In 
general form and external markings this shell closely resembles the common Car- 
boniferous species, Orthis Michelini of Leveillé (O. filiaria, Phill.), the only obvious 
distinction being the slight depression near the umbo of the deeper valve, but the form of 
the muscular impressions in the dorsal valve affords a sufficient distinction ;” also that ‘“ the 
specimens figured by M. Rouault are much distorted, and only show the exterior, for 
which reason the species is refigured here. It is interesting as a connecting link with the 
Lower Silurian strata of Brittany, and should teach us caution in assigning an age to 
1 At p. 68 of his memoir M. Rouanlt describes his O. Berthoisi in the following words :— 
“* Yappelle particuliérement l’attention des savants sur ce fossile tant a cause de la rareté de ces Mollusques 
dans ces Schistes (de Couyére en Bretagne) que des caractéres qu’il va nous offrir. C’est une grand coquille 
du groupe des Arcuato-striate de M. de Verneuil; elle est peu epaisse et sa valve dorsale seule est sensiblement 
renflee et munie d’un crochet peu saillant; ses deux valves sont couvertes de stries fines, tres nombreuses, 
et de lignes d’accroissement trés marquées; mais le caractére le plus saillant et en méme temps le plus 
difficile 4 préciser dans une description specifique nous est fourni par les nombreuses deformations qu’elle 
asubies. Ces deformations sont telles que sur soixante échantillons il ne m’a pas été possible d’en ren- 
contrér deux de formes semblables, bien qu’ils appartiennent & une espéce dont la forme symeétrique est bien 
reconnue.” 
? Mr. Sharpe’s description of O. Berthoisi is as follows :— Shell ovate, with a very short hinge-line ; 
dorsal valve slightly convex ; ventral valve more convex than the dorsal, with a slight mesial depression, 
which, beginning at the umbo, dies out before reaching the middle of the valve ; both valves are covered with 
very numerous fine bifurcating ribs, which divide many times before they reach the margin, and are crossed 
at irregular distances by strong concentric lines ; lateral ribs curved. Length 1 inch, greatest breadth below 
the middle ¢th of an inch.” 
