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EOCENE MOLLUSCA. 



the middle of the whorls they become more prominent, thicker, and irregular, 

 very slender lines occasionally intervening between the larger ones : the last whorl is 

 short, and terminates in front in a moderately long, narrow, and nearly straight canal. 

 The aperture is obovate ; the outer lip much arched, thin, sharp-edged, and most 

 generally smooth and simple within, although in young specimens from Bramshaw 

 the outer lip is occasionally plicated ; the sinus, which is on the shoulder, is moderately 

 wide, deep, and triangular in form. 



The present species is widely spread, and in England ranges from the London 

 Clay to the fluvio-marine deposits of Headon Hill ; it is very variable, almost every 

 locality presenting some modification of the dimensions or characteristic ornamentation 

 of the shell. The typical forms described by Sowerby as P. plebeia are confined' 

 to the middle Eocene deposits ; in the older deposits the species is represented by two 

 varieties. In the first of these, var. longcp-va, from Highgate, the shell is narrower, 

 and the posterior margins of the whorls, as well as the tubercles, are ornamented 

 with very slender raised lines ; this variety is narrower than the shells from the 

 Bolderburg, referred by Nyst to Basterot's species, but it agrees with them in all 

 other respects, particularly in the peculiar modification of the transverse lineation 

 which characterises the latter shells. In the other variety, macrobia, from Clarendon, 

 the spire is shorter, the posterior margins of the whorls are smooth, or they only present 

 one or two feeble concentric lines, and the tubercles on the shoulders are more distant, 

 larger, and coarser than in. the typical form. In this modification the shells agree 

 with those from Bos d'Arros, forming Rouault's var. D of P. denticida; and with it 

 I should also have unhesitatingly associated the shells from the same locality consti- 

 tuting that author's species P. subcarinata, were it not for the different condition 

 attributed to them of the embryonal whorls, of which the last two are described 

 as being smooth instead of longitudinally ribbed. But for this distinction, the 

 mature shells could not be satisfactorily separated from those forming the var. 

 D of P. denticida, the slight differences which exist in the condition of the trans- 

 verse lineation and of the tubercles not being, of themselves, of specific value. 

 May not the smooth surface be due to disintegration, the outer layer of shelly matter 

 in the pullus being, in general, more susceptible of decomposition than that in the 

 mature shell? Recognising, however, the value of the character pointed out by 

 Rouault, I have cited his species with doubt. 



Among the foons found in the middle Eocene deposits at Bracklesham Bay, 

 Brook, and Bramshaw, are those constituting the variety gracilenta ; in these the 

 shell is smaller and slenderer, the concentric lines are acute and nearly even, and 

 the tubercles are neither so wide nor so prominent. This variety presents a close 

 resemblance to some shells from Cuise-Lamotte (Sab. inf.), presented to me by 

 M. Deshayes, and by him named P. denticidata. 



In the upper Eocene deposits at Lyndhurst, Hordwell, Colwell Bay, and Headon 



