RHYNCHONELLA. 77 



or dental valve less inflated than the opposite one, with a shallow sinus ; beak short, acute, 

 and moderately incurved; foramen small, almost contiguous to the umbo, and entirely 

 surrounded by the deltidium and its tubular expansions ; beak ridges well defined, leaving 

 a flattened space or false area between them and the hinge line. Dorsal or socket valve, 

 generally more gibbose than the opposite one, its uniform convexity being interrupted 

 from about the middle of the valve to the front, by a widely slightly produced mesial fold. 

 The hinge line of the ventral valve indents the lateral portions of the umbo ; margins 

 flexuous on approaching the front, become sharply bent at almost right angles, indenting 

 to a considerable extent the frontal edge of the dorsal valve. Externally, each valve is 

 ornamented by from fifty to sixty plaits; these commonly, on approaching their terminations, 

 become flattened, and, as if divided by a narrow longitudinal split or depression. In the 

 interior of the dorsal valve, two curved processes exist for the support of the spirally coiled 

 extensile arms. Shell structure impunctate. 1 



Dimensions variable : length 12, width 15, depth 9 lines. 



j 5 11} ,, 1*> »> 12 ,, 



Var. octoplioata, Sow. Plate X, figs. 1 — 17. 

 This variety agrees in general character with R. plicatilis (type), but differs more often 

 by its plaits, which, on approximating, the front and lateral margins unite two by two, 

 forming fewer and larger costae ; these last are also commonly acute, and not flattened or 

 split, as is very often the case in the typical specimens of R. plicatilis. 



Var. Woodwardii, Bav. PI. X, figs. 43 — 46. 



Tisiiebratuxa Gallina, Woodward. An Outline of the Geol. of Norfolk, tab. iv, 



fig. 12, 1833. 



Diagnosis. Shell transversely oval : valves moderately convex, with a shallow sinus in 

 the ventral, and slightly produced mesial fold in the opposite one. Externally each valve 

 is ornamented by from 24 to 44 simple plaits, often split close to the margin ; length 9, 

 width 10 to 12, depth 6 to 7 lines. 



05s. It was justly observed by the author of the 'Pal. Erancaise,' 3 that Sowerby's 



1 Dr. Carpenter has described and figured the remarkable shell structure observable in this species, in 

 his very valuable memoir " On the Microscopic Structure of Shells." ('British Association' for 1844, 

 pi. xiv.) See also the " Introduction " to this work, pi. v, fig. 6. 



2 'Terrains Cretaces,' vol. iv, p. 58, 1847; but prior to M. D'Orbigny, Dr. Mantell had stated 

 1822, that R. octoplicata was only a var. of E. plicatilis, "the specimens in my possession vary so much 

 in the number of plica, and the convexity of the valves, and the characters of each are so equally blended, 

 in many examples, that I have beeu obliged to consider them as only a variety of the same species (Fossils 

 of the South Downs). Mr. Morris and Dr. Bronn have likewise arrived at similar conclusions, but we 

 cannot admit all the synonyms mentioned by the learned German author, viz., T. latissima, parvirostris, 

 Martini, and nuciformis'' Geinitz likewise looks upon R. octoplicata, as a var. of R. plicatilis, but errone- 

 ously adds T.pisum, Sow. and T. Mantelliana, Sow., two well-distinguished species. 



