BRITISH CRETACEOUS BRACHIOPODA. 



differences, proving modifications in the dispositions of the animal relative to the shell. 

 Many persons from not having devoted sufficient attention to these most interesting 

 variations, and from contenting themselves by the superficial observation of some 

 external characters, would place together many animals essentially different in their 

 details, while separating others whose internal organisation is similar; but those who 

 have truly investigated the matter, after long and conscientious observations, all now admit 

 the necessity of subdividing the few families composing the class into a certain number of 

 genera or subgenera. 



I regret not having always been able to examine the interior of the species, and 

 therefore remain in some cases uncertain in which section such forms should be placed. 1 



Genus — Lingula, Brvguiere. 1789. 



Shell inequivalved, one valve more convex than the other, more or less oval, elongated, 

 tapering at the beaks, widened at its palleal region ; without hinge, valves held together by 

 the adductor muscles; attached to submarine bodies by a long muscular pedicle issuing 

 between the beaks ; a groove, existing for its passage in that of larger valve ; arms 

 fleshy, without any calcareous support ; structure horny, covered by an epidermis ; two 

 muscular impressions in the one, and four in the other valve. 



1. Lingula truncata, Sow. Plate I, figs. 27, 28, and 31. 



Lingula truncata, Sow. in Fitton. Observations on some of the Strata below the 

 Chalk, read before the Geol. Soc. in 1827, printed in vol. iv, 

 pi. xiv, fig. 15, of the Trans, of the Geol. Soc., 1836. 



— — Morris. Catalogue, 1843. 



— — Forbes. Catalogue of L. G. Sand Fossils, Quart. Journ. of the 



Geol. Soc, vol. i, p. 346, 1845. 



— — Fitton. Strat. Section, Quart. Journ. of the Geol. Soc, No. 11, 



p. 289, 1847. 



— Rauliniana, D'Orb. Pal. Franc. Ter. Cretace'es, vol. iv, p. 80, pi. 490, 1847. 



— truncata, Bronn. Index Pal., p. 656, 1848. 



— — D'Orb. Prodrome, vol. ii, p. 84, 1850. 



1 I would recommend to the study of all scientific observers the instructive and valuable work by 

 Professor Milne-Edwards, bearing for title • Introduction a la Zoologie Generale, ou Considerations sur les 

 Tendances de la Nature dans la Constitution du Regne Animal.' Paris, 1851. 



