An jEolian Dcjjosit at Clevedon. 365 



anteriorly only two very faint grooves, which by stronger develop- 

 ment produced the trilobation of the cardinal process of Leptestia ; 

 the differences of the ventral valves are still more pronounced, as : 

 (1) the shell-callosities of Leptestia in the visceral fields ; (2) its shell 

 thickenings or ridges around the visceral fields ; (3) its surface 

 structure. 



Plectambonifes llandeiloensis (Davidson)^ is related to Leptestia, as 

 well as to P. leptelloides, especially by its slightly similar visceral 

 fields of the dorsal valve. Davidsonia verneuUiana Bouchard ^ 

 has muscular impressions in the umbonal cavity with almost the same 

 arrangement. The conical shell callosities and spiral markings of 

 Davidsonia are absent in Leptestia. Davidsonia does not possess 

 dental lamellae and a septal process. The dorsal valves differ more 

 considerably ; the exterior adductors of Leptestia enclose only a 

 portion of the interiors, while the interior pair is enclosed by the 

 exterior in Davidsonia. The cardinal process differs also. It might 

 be suggested that Leptestia is an ancestral form of the middle 

 Devonian Davidsonia, and the differences are naturally due to the 

 mode of life {Leptestia is a free form, while Davidsonia is fixed by its 

 ventral valve), as well as to the great lapse of time. 



An Aeolian Pleistocene Deposit at Clevedon. 



By Edward Greenly, D.Sc, F.G.S. 



[This paper was read before the Geological Society of London, 

 and discussed, on 1st December, 1920. Subsequently, however, 

 I asked and obtained leave from the Council to withdraw it from 

 publication, m order to relieve the pressure on the Quarterly Journal, 

 as has been announced in Proc. Geol. Soc, 1920-1, p. iv, footnote.] 



Introductory. 



General Aspect. 



Petrological Characters. 



Structures. 



Pliysical Features of the District. 



Distribution and Relations of the Deposits. 



Palaeontology. 



Local Detail. 



The Age of the Deposits. 



The Origin of the Dejjosits. 



The Loess — a Comparison. 



The Contemporary Levels. 



Sources of the Materials. 



Recapitulation. 



Introductory. 

 rPHE deposits which are the subject of this paper have already 

 -^ attracted some attention, because of their connexion with the 

 contents of a small, though highly fossiliferous bone-cave at a place 



^ F. R. C. Reed, " Ord. and Sil. Brach. Girvan district " : Trans. Boy. 

 Soc. Edinburgh, vol. li, 1917, pt. iv, p. 376, pi. xiii, figs. 32-4 ; pi. xiv, fig. 1. 



^ T. Davidson, Brit. Dev. Brach.iGpoda, 1865, p. 74, pi. xi^ figs. 13-16 ; pi. xv, 

 fig. 18. Hall, J., & Clarke, J. M., Pal. New York, vol. viii, p. 301, pi. xvA, 

 figs. 31-3. 



