BUDLEIGH-SALTERTON PEBBLE-BED. 357 



series than that of the slates of La Couyere, and 1 still consider it a smaller variety of 

 0. Berthoisi, and to which I have given the varietal name of erratica. 



In the memoir, ' Congres de Nantes,' 1878, and at p. 506 of the 'Bulletin of the 

 Geological Society of France,' 3rd ser., vol. iv, 1876, MM. de Tromelin and Lebesconte 

 admit the varietal name of erratica for the shell under description. M. de Tromelin 

 states that the fossil is extremely rare in the quart zite of May, having found but a single 

 example, while at St. Germain-sur-Ille it is exceedingly abundant in the shape of internal 

 casts and impressions of the exterior, but almost every example is compressed and out of 

 its natural shape from fossilisation. This variety, according to M. de Tromelin, and in 

 this view I fully coincide, entirely agrees, with that found in the Portuguese rocks 

 above mentioned. The ventral valve in many well-preserved specimens of the var. 

 erratica is almost flat, with or without a slight depression, commencing about the middle 

 of the shell, and extending to the front. 



39. Orthis pulvinata, Salter. B. S. Sup., PI. XLI, figs. 10, 11. 



Orthis pulvinata, Salter. Quart. Journ. Geol. Soc., vol. xx, p. 294, pi. xvii, fig. 8, 



1860. 

 — Dav. Quart. Journ. Geol. Soc, vol. xxvi, pi. v, figs. 17 — 19, 1869. 



Transversely oval ; hinge-line shorter than the width of the shell ; valves uniformly 

 and moderately convex ; surface marked by fine, radiating, thread-like striae, increasing in 

 number towards the margin by the interpolation of shorter strias. Area narrow ; the 

 internal cast of the ventral valve shows a large saucer- shaped muscular area, divided by 

 a widish, slightly-raised, flattened ridge. In the internal cast of the dorsal valve there is 

 a narrow longitudinal split, margined by a raised, rounded rim, under which are placed 

 in pairs, one above the other, the quadruple elevations due to the adductor muscle, and 

 longitudinally separated by a narrow concave space (see PI. XLI, figs. 11 and 11a). 

 Dimensions variable. 



Length 11, breadth 14 lines. 



Obs. — The internal cast of the dorsal valve, the one we have drawn, was in 1863 

 described and figured by Salter, the exterior and interior of the other valve being 

 unknown to him at the time ; subsequently, however, impressions of the exterior, as well 

 as the internal cast of the ventral valve, were found by Mr. Vicary, so that it became 

 possible to draw up a complete description of the species. 



M. de Tromelin and some other palaeontologists seem disposed to consider 0. pulvinata 

 a transverse form of 0. Berthoisi, var. erratica ; but when you compare the figures of the 

 interior of the dorsal valve (fig. 11 a) with that of 0. Bert/ioisi,vav. erratica (fig. 8 b), a 

 great difference is at once discernible. I am also disposed to consider it probable that 



47 



