288 KOCENE 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. pledcia are confined 
to the middle Eocene deposits ; in the older deposits the species is represented by two 
varieties. In the first of these, var. /ongeva, 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, macrodia, 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. denticula; 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. denticula, 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, | have cited his species with doubt. 
Among the forms 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. denticulata. 
In the upper Eocene deposits at Lyndhurst, Hordwell, Colwell Bay, and Headon 
