EXPLANATION OP PLATE XX. 



PlacaMyln^ bivaricos)is, Gaskoin, sp., var. solulus, Eth. fil. 



Pinf. 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 pos- 

 terior margin very apparent, and the thickened margins with 

 concentric laminae equally so. 



,, 4. Side view of the entire .specimen Pig. 3, with the umbilicus parti- 

 ally disclosed. 



,, 5. Side view of a somewhat less thickened example with the umbilicvis 

 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. 



Placostylua bivaricosus, Gaskoin, sp. 



Pig. 7. A recent example showing a transition towards the var. solidvs, 

 and the twist in the pillar or columellar lip more marked than 

 in Fig. G. 



Figs. 1 - .3 from the Coral-sand Rock ; Pig. C from the Sand-dunes 

 Pig. 7 Recent . 



[Prom drawings by Mr. G. H. Barrow, Australian Museum.] 



