RHYNCHONELLA. 91 



of Rhynchonella, and, had space permitted, illustrations could have been introduced 

 exhibiting every passage uniting such shells as figs. 28 and 30. In all, the umbo of the 

 dorsal valve is much incurved, its extremity being to a lesser or greater extent concealed 

 under the development or encroachment of the deltidium. On well preserved examples 

 the concentric lines of growth are numerous and in close approximation, giving to the 

 upper ridge of each plait a somewhat granulated appearance, but which is more deceptive 

 than real, since these projections form part of an uninterrupted and continuous concentric 

 line or ridge. In young individuals, no trace of sinus or mesial fold can be perceived, the 

 frontal line being straight, but with age both the sinus and fold gradually appear, and 

 always exist to a greater or lesser degree in adult individuals. Geinitz published two 

 figures representing the exterior of the ventral valve of a shell he terms Ter. Triangularis} 

 and which in external contour appears to somewhat resemble our British examples, but the 

 profile view would almost indicate a different species. M. D'Orbigny's figures 2 of his so- 

 termed Bh. depressa do not appear to resemble Sowerby's shells, and belong (I have little 

 doubt) to a distinct species, although the description published in the ' Pal. Erancaise,' 

 would denote a shell different from that figured in his plate. 



B. depressa abounds in the Upper Green Sand of Farringdon, along with B. nuciformis . 

 Plate XI, fig. 28. A specimen of B. depressa, Sow., from the Upper Green Sand of 



Farringdon, in the cabinet of Mr. Lowe, it presents 11 plaits on 

 the mesial fold ; 28 s c are enlarged representations. 

 „ fig. 29. Another example from the same locality, in which the central plaits 



are narrower than the lateral ones. 

 ,, fig. 30. A specimen with an unusually small number of plaits, from the 



collection of Mr. Sharpe ; fig. 3% enlarged. 

 ,, fig. 31. A young individual, from the same locality. 



,, fig. 32. A young and somewhat elongated example, believed by Mr. Sharpe 

 to represent B. triangularis; fig. 32* a magnified illustration, 

 from the collection of Mr. Sharpe. 

 Plate XII, fig. 26. A large transverse specimen, in which the beak is not so much 



produced as in those figured in PL XI, locality Farringdon. 

 In the Upper Green Sand of Warminster, in that of the Isle of Wight, and in equiva- 

 lent beds at Chardstock, are found numerous examples of two forms represented in PI. XII, 

 figs. 28 and 30. They appear to constitute (if not separate species) well-marked varieties 

 of Bh. depressa of Farringdon. I will therefore briefly mention them under the head 

 of varieties a and b. 



1 ' Charact. der Schichten und Petref.,' pi. xix, figs. 1 — 3, 1842. 



2 'Pal. Franc., Terrains Cretaces,' vol. iv, p. 18, pi. 491, figs, 1 — 7. M. D'Oibigny states that his 

 specimens were obtained in the Terrain N6ocomien of France. 



