RHYNCHONELLA. 93 



42. Rhynchonella. nuciformis (Sowerbg, Sp.). Plate XI, figs. 23 — 27, and Plate XII, 



fig. 27. 



Terebratula nuciformis, Sow. Min. Con., vol. v, p. 166, tab. 502, fig. 3, 1825. 



— — Woodward. A Synopt. Table of British Org. Remains, 



p. 21, 1830. 



— — Morris. Catalogue, 1843. 



— — Tennant. A Strat. List of British Fossils, p. 47, 1847. 



— — Austen. Quart. Journ. of the Geol Soc., vol. vi, p. 477, 



1850. 



— — Sharpe. Quart. Journ. of the Geol. Soc, vol. x, p. 192, 



1853. 



Diagnosis. Shell more or less transversely oval and inflated : valves unequally convex, 

 the dorsal one more often the deepest : beak acute, moderately produced and incurved : 

 foramen almost contiguous to the umbo, of moderate dimensions, and entirely surrounded 

 by the tubular prolongations of the deltidium : between the beak ridges and hinge line 

 exists a flattened space, which slightly indents the lateral portions of the umbo. 



The ventral or dental valve presents a longitudinal depression or shallow sinus, to which, 

 in the opposite valve, a mesial fold corresponds of variable elevation : externally each valve 

 is ornamented with from 30 to 40 plaits, 7 to 12 occupying the mesial fold or sinus, the 

 ridges of the plaits are more or less acute, but, on approaching the front and lateral margins, 

 often become flattened, with a longitudinal indented line along their centre. Dimensions 

 and relative proportions very variable : length, 6^, width, 7, depth, 7 lines. 



7 9 6 



Obs. Bh. nuciformis was stated by Sowerby to be a globose shell, smaller than a hazel- 

 nut, the edges of the plaits being rounded, and near the front often toith a sunk line upon 

 them (loc. Farringdon) ; and although distinguished in England from other Bhynchonella;, 

 has, on the Continent, been very generally confounded with other forms. M. D'Orbigny 

 places it as a synomyn of B. depressa (Sow. sp.), 1 but from which it appears to differ by its 

 general shape, which is transversely or oblongly oval, and at times almost circular, with its 

 plaits often split near the front and margins, as is so well exemplified in the Palasozoic Bh. 

 Wilsoni and other similar forms. While B. depressa (Sow.), as its name implies, is a 

 depressed shell with imbricated plaits, this last character not having been observed in true 

 B. nuciformis. Prof. Bronn commits another mistake, by considering the shell we are 

 describing to be the same as B. plicatilis, 2 from which it appears removed by more than 



1 The author of the 'Pal. Franc.,' does not appear to have been acquainted with Sowerby's R. nuci- 

 formis and depressa, for his figure of this last ('Pal. Franc.,' vol. iv, pi. 491, figs. 1, 7), does not agree with 

 any of the examples found in England. 



2 'Index Pal.,' vol. ii, p. 1246. 



