THE NAUTILUS. 89 



dddrl. Similar, but with a rounded sinus in the 

 outer lip above ; green. 



P. GRACILIS VIRIDIS n. V. 



The operculum of P. miiltii-arlnatus Miller has more whorls than 

 those of the other species, and may eventually be placed in a new 

 genus. 



P. bisiiiuat'iis might be regarded as an immature stage of P. 

 Sykesii 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. diiiochilus closely resembles P. microthauma 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. Bmhii 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. 1 



1 Since 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. " Potatnolitlms 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, 

 the same species. There is only one tablet, and this contains one of your var- 

 iety \_stifersulcattis] mingled with the rest, as also one specimen which is not 

 the same species but may be Pctitiana. 



" P. Sykesii. I 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." 

 [1 had supposed d'Orbigny's shells were mature, and therefore separated 

 Sykesii on the ground of its peculiar peristome. It remains to be seen whether 

 adult Pctitiana will prove to have the same characters, but I agree with Mr. 

 Sykes that it is likely]. 



P. gracilis. This is distinct from ficinm, which is a thinner and slighter- 

 built species, [and does not show the same apertural characters.] 



