RHYNCHONELLA. 87 



developed, and the plaits always smaller and more delicate than those observable on the 

 Cambridge shell, which it has been agreed to term R. sulcata. 



Few species vary more in external shape or detail than the one under consideration, 

 as may be seen from the series of illustrations I have selected from among several hundred 

 individuals assembled from a single locality by Mr. Carter. The mesial fold and sinus 

 does not always occupy the middle of the shell, nor in all cases is it symmetrical, for out 

 of ten examples eight or nine will have their fold and sinus shifted more to the one or the 

 other side, as seen in figs. 23, 25, and 27 of our Plate, while in some examples the one 

 half of the valve is more elevated than the other, being twisted indifferently to the right or 

 to the left, as is so common to Rh. inconstans, and to those malformations of R/i. compressa 

 to which Sowerby had applied the term R. dimidiata. The plaits are generally simple, 

 but in some instances, although rarely, bifurcate here and there. 



Rh. sulcata abounds in the Upper Green Sand near Cambridge, is less commonly met 

 with in the neighbourhood of Warminster, and was found by Mr. Bean in the Speeton Clay 

 of Yorkshire. Some rare examples have likewise been found in the Gault of Folkstone, and 

 in the corresponding bed at Wissant, on the French coast. M. D'Orbigny mentions the 

 species as abounding in his Terrain Albien at Grandpre, and Fleville (Ardennes), 

 Gerodot (Adsc), at thePerte du Rhone (Ain), and Clausayes (Drome), &c. 



Plate X, figs. 18 — 20, and 23 — 36. Illustrate a series of specimens from the Upper 



Green Sand of Cambridge, in the cabinet of Mr. Carter. 

 Figs. 18, 21, are regular in shape, the others show some 

 of its innumerable malformations. Figs. 15 and 36 are 

 internal casts, on which the muscular and vascular impres- 

 sions are well preserved. 

 ,, figs. 21 — 22. From the Speeton Clay, in the collection of Mr. Bean. 



39. Rhynchonella Mantelliana, Sowerby, Sp. Plate XII, figs. 20 — 23. 



Terebratula Mantelliana, Sowerby. Min. Con., vol. vi, p. 72, tab. 537, fig. 5, 



1825. 



— — Fleming. A Hist, of British Animals, vol. i, p. 374, 1828. 



— — V. Buck. Mem. Soc. Geol. de France, vol. iii, p. 154, 



pi. xv, fig. 26, 1838. 

 _ _ IGeinitz. Char. Kreid., p. 15, 1839. 



— — Morris. Catalogue, 1843. 



Rhynchonella Mantelliana, D'Orbigny. Pal. Franc.. Ter. Cretace's, vol. iv, p. 40, 



1847, (the illustrations given by this author, pi. 498, 

 figs. 1 — 5, do not recall the common aspect of the 

 Sowerby species.) 



Diagnosis. Shell transversely obovate, rather wider than long; valves almost equally 



12 



