EXPLANATION OF PLATE XX. 
Plaeo stylus bivaricosus , Gaskoin, sp., var. solidus, Eth. fil. 
Fig. 1. Front view of a much thickened individual, with the emargination 
on the inner edge of the outer lip acute, the posterior channel 
well marked, and the tubercle of the callosity obtusely rounded. 
„ 2. Back view of the same specimen, showing the coarse sculpture 
and irregularly crenulated sutures. 
„ 3. Peristome and portion of the last whorl of a thicker-shelled indi- 
vidual, the emargination of the outer lip is shallow, but the 
tubercle very prominent, the false denticulation on the -pa*- 
terior margin very apparent, and the thickened margins with ' 
concentric lamin* equally so. 
„ 4, Side view of the entire specimen Fig. 3, with the umbilicus parti- 
ally disclosed. 
„ 5. Side view of a somewhat, less thickened example with the umbilicus 
practically closed. 
„ G. Peristome and portion of last whorl of a much smaller specimen 
from the Sand-dunes, with the general varietal characters 
marked in less degree, but showing a tendency towards a 
twisted columella. 
Placostylus bivaricosus > Gaskoin, sp. 
Fig. 7. A recent example showing a transition towards the var. solidus , 
and the twist in the pillar or columellar lip more marked than 
in Fig. 6. 
Figs. 1-5 from the Coral-sand Rock ; Fig. 6 from the Sand-dunes 
Fig. 7 Recent. 
[From drawings by Mr. G. II. Barrow, Australian Museum.] 
