30 CRETACEOUS LAMELLIBRANCHIA. 



more distinctly smaller than those on the anterior part, and by the presence of 

 the small rib at the bottom of each groove. See also L. farringdonensis (below). 



L. expansa, Forbes, 1 from the Hythe Beds of Hythe, is known to me only from 

 the type specimen which is preserved in the Museum of the Geological Society 

 (No. 2056). It is an internal cast in clay, somewhat crushed, and shows the 

 ribbing only imperfectly. I think it is probably an example of L. parallela, but 

 more specimens from the same horizon are needed before a confident opinion can 

 be given. Similar remarks apply to L. lingua, Forbes, 2 which comes from the same 

 horizon and locality, and is likewise preserved in the Museum of the Geological 

 Society (No. 2058). 



This and the following eight species are provisionally referred to the sub-genus 

 Mantellum, with which they agree in the form of the shell and, in many cases, in 

 the general character of the ornamentation. They differ, however, from the type 

 of Mantellum in having the valves closed or almost closed, but there is, as Phillip! 

 has pointed out, every transition from the species in which the valves gape widely 

 to others in which they are closed. 



Remarks. This species shows a fair amount of variation in the proportions of 

 length and height, and also in the obliquity of the shell. 



The type-specimen of L. parallela is an internal cast, and consequently all 

 writers have found it practically impossible to make out the characters of the 

 species from Sowerby's figure. A comparison of the type with better preserved 

 specimens leaves no doubt in my mind that Sowerby's species is really identical 

 with the form described by d'Orbigny as L. Cottaldina. The latter author 

 referred a species found in the Gault (L. gaultinn, p. 31) to L. parallela, Sowerby. 



Types. The type is from the Hythe Beds of Maidstone and is preserved in the 

 British Museum (No. 43,292). The specimen from Upware figured as L. farring- 

 donensis by Keeping is in the Sedgwick Museum, Cambridge. 



Distribution. Pmia-bed and Atherfield Clay of Atherfield. Ferruginous Sands 

 of Shanklin. Hythe Beds of Hythe, Lympne, and Maidstone. Sandgate Beds of 

 Sevenoaks. Folkestone Beds of Folkestone. Lower Greensand of Faringdon and 

 Upware. Speeton Clay of Speeton. 



LIMA (MANTELLUM) FARRINGDONENSIS, Sharpe, 1853. 



1853. LIMA FARRINGDONENSIS, D. Sharpe. Quart. Journ. G-eol. Soc., vol. x, p. 198, 



pi. vi, fig. 2. 



Non 1883. W. Keeping. Foss., etc., Neoc. Upware and Brick- 



hill, p. 112, pi. v, fig. 12. 



1 ' Quart. Journ. Geol. Soc.,' vol. i (1845), p. 249, pi. iii, fig. 11. 



2 Ibid., p. 249, pi. iii, fig. 10. 



