23G EOCENE MOLLUSCA. 



quite smooth, except where two slender, faintly crenulated, raised lines form a border 

 round the suture. The keels are band-like and simple ; the surface of each is traversed 

 by a shallow but conspicuous furrow ; the edge of the shoulder-keel is sharp, smooth, 

 and bent upwards like that of the keel which characterises the preceding variety. The 

 space between the two keels and the surface of the front part of the whorls is orna- 

 mented with numerous narrow, unequal, round-edged bands, between some of which 

 a slender line occasionally appears ; these bands are smooth, or but slightly roughened 

 by the lines of growth. 



A form intermediate between and connecting the last two varieties occurs, in which 

 the sharp, smooth-edged, and furrowed shoulder-keel of the variety Pagoda is associated 

 with an obscurely plicated anterior keel, and faintly granulated spiral lines. 



Although the shell for which the variety Pagoda is proposed, differs so much in the 

 ornamentation from the typical form, I do not regard it as specifically distinct. In the 

 several varieties described, the characteristic ornaments of the species have been 

 modified, and have become gradually less prominent, until at last, in the variety 

 revoluta, they are almost obliterated ; and the intermediate form, before noticed, con- 

 nects that variety with the one in question, which, in fact, merely presents in strength a 

 character of ornament which has already appeared in other varieties. 



All the varieties are rare except the varieties gyrata and Pagoda, which appear to 

 be more abundant. 



Size. — Of the var. ditropis — axis, 5-12ths of an inch ; diameter, 2-12ths of an inch : 

 of the var. concinna and pidcherrima — axis, 7-12ths of an inch; diameter, rather 

 more than 3-12ths of an inch: of the var. gyrata — axis, 1 inch, nearly; diameter, 

 rather more than 5-12ths of an inch : and of the var. Pagoda, if the shell were per- 

 fect — axis, 1 inch and 2-12ths, nearly; diameter, 5-12ths of an inch. 



Localities. — For the var. ditropis — Highgate, Potters' Bar, Hornsey, Hampstead; 

 for the var. concinna — Highgate, Potters' Bar, Hornsey, Finchley ; for the var. gyrata — 

 Hornsey, Southampton, Shenfield ; for the var. pidcherrima — Highgate and Clarendon 

 Hill; and for the var. revoluta and Pagoda — Clarendon Hill. French — Parnes, 

 Grignon, Beyne, Mouchy-le-Chatel, Compiegne (fide Desh.), Chaumont, Amblainville, 

 Cuisse-Lamotte, Pont-Sainte-Maxence (fide Graves). 



The specimens, fig. 10 b and k, are from Mr. Wetherell's cabinet. 



No. 158. Pleurotoma microdonta, F. E. Edwards. Tab. XXVII, fig. 4. 



P. testa elongatd, fusi/ormi, tuber culatd, spiraliter exilissime lineatd: spird elevatd, 



h -cofiicd : anfractibus angulatis,unicd serie tuberculorum denti-formium munitis ; margini- 



bus posticis talis, pauxillum cavatis, ad suturam cingulo triplici, distanter plicato, cinctis .- 



