THE NAUTILUS. 89 
dddd. Similar, but with a rounded sinus in the 
outer lip above; green. 
P. GRACILIS VIRIDIS 0. Vv. 
The operculum of P. multicarinatus Miller has more whorls than 
those of the other species, and may eventually be placed in a new 
genus. 
P. bisinuatus might be regarded as an immature stage of P. 
Sykesti were it not that until the lip expansion of the latter is fully 
developed no trace of sinuation occurs, the sinuses being developed 
in the thickened margin beyond the expansion. 
The variety of P. lapidum described and figured by Strobel 
(Mater. Malac. Argent.) from a single shell, does not seem to have 
sufficiently tangible characters for recognition as distinct from typi- 
cal lapidum. 
P. dinochilus closely resembles P. microthuwna in characters of 
the lip varix and aperture, and it may possibly prove to be a form 
of that species when extensive series of each are collected; but the 
other features of the shells are so strikingly different and so con- 
stant in the series before me, that their union would not be justified 
with present knowledge. 
Certain forms of P. Bushii have two weak keels on the back and 
offer an approach to P. tricostatus, and the two may prove to be 
specifically the same, although proof is lacking that this is the case. 
In P. Buschii the keels or sulcus on the back are weaker when 
present, the umbilical crescent is larger and angular, and the form 
less elevated." 
1Since the above table has been in type, I have received Mr. E. R. Sykes’ 
notes on certain species which he was so kind as to compare at my request, 
with d’Orbigny’s types in the B. M. “ Potamolithus lapidum. Compared with the 
typical series your shells differ a bit in the aperture being somewhat pyriform, 
while those of the museum series are more rounded. Still they are, I think, 
thesame species. There is only one tablet, and this contains one of your var- 
iety [swpersulcatus] mingled with the rest, as also one specimen which is not 
the same species but may be /e/idiana. 
“ P, Sykesii, 1 think that this is only a form of Petitiana; there are how- 
ever only two specimens, both immature, of this last species in the museum.” 
[I had supposed d’Orbigny’s shells were mature, and therefore separated 
Syesii on the ground of its peculiar peristome. It remains to be seen whether 
adult Petitiana will prove to have the same characters, but I agree with Mr. 
Sykes that it is likely]. 
P. gracilis. This is distinct from péciem, which is a thinner and slighter- 
built species, [and does not show the same apertural characters. | 
