PRESTWICH WOOLWICH AND READING SERIES. 75 



as well as the regular surface and distant ribs, distinguish it from 

 the S. concentrica of Portlock, a fine species, which occurs also in 

 the Horderley section, and which from good specimens I find to be 

 truly distinct from S. compressa of the Caradoc sandstone. 



Localities. Near Hope Bowdler and Cheney Longville, abundant 

 in the thin flags (4) of the Horderley section. 



4. NuCULA VARICOSA, n. sp. 



N. deltoidea, subsequilateralis, utraque rotundata. Umbo subcentralis, ele- 

 vatus, falcatus, lunulam excavatam imminens. Margo anticus cardinis 

 arcuatus ; margo ventralis convexus. Testa supra medium modice con- 

 vexa nee gibba, Isevis, nisi sulcis 5-8 concentricis gradatis exarata. Sulci 

 aut varices in juvene remoti, ad marginem gradatim approximati. Long, 

 lin. 4, lat. 4. 



This pretty species is unlike any other palaeozoic Nucula with 

 which I am acquainted, by the few sharp concentric lines, or rather 

 steps, of growth, which are more approximate in the older parts of 

 the shell. It is most abundant in the Bala rocks, and has been re- 

 ferred by Prof. M'Coy to the N. levata. Hall, which has only faint 

 concentric lines of growth, a pointed anterior side, and a much less 

 prominent beak. We have never seen it from the Upper Silurian. 



Localities. Fine specimens occur in the uppermost calcareous 

 beds of No. 4 of the above section ; — Onny River ; at Acton Scott ; 

 Ticklerton, &c. Also Bala ; Conwav Falls, &c., North Wales. 



On the Structure of the Strata between the London Clay and 

 the Chalk in the London and Hampshire Tertiary Systems. 

 By Joseph Prestwich, Jun., Esq., F.B.S., F.G.S. 



Part II. — The Woolwich and Reading Series. 



[Plates I. II. III. IV.] 



[Read May 18, 1853. For the other Communications read at this Evening Meeting 

 see Quart. Journ. Geol. Soc. vol. ix. p. 274,] 



On two former occasions I have given some account of the de- 

 posit immediately underlying the London Clay, as well as of that 

 which, to the eastward of London, lies upon the Chalk, and which I 

 have respectively termed the " Basement Bed of the London 

 Clay*" and the "Thanet SANDsf." Between these divisions, 

 which form the upper and lower portions of the Lower London 

 Tertiaries, is a group of sands, pebble beds, and mottled clays, 

 extending from Sandwich to Marlborough and from Newhaven to 

 Dorchester. This group, with the two above-mentioned, completes 

 the series of these Lower Tertiaries, and is the one which more 

 particularly embraces the beds which have hitherto been described 

 as the "Plastic Clay Formation,'' exhibiting in one part of its range 

 the mottled clays of Reading and Newbury, and, in another, the 

 clays and sands, with fluviatile and sestuarine shells, of New Cross, 



* Quart. Journ. Geol. Soc. vol. vi. p. 252. t Jl'td- vol. viii. p. 235. 



