88 Proceedings of the Royal Society of Victoria. 



P. bassi shows some affinity with P. bnyrichi, Hilgendorf, in 

 that the fascicle is broad, and is situated a little below the middle 

 of the body-whorl, and tlie slit is short, in these and other 

 respects it appears to agree with Fischer's section Perotrochus, 

 but it is distinctly umbilicate. Thus if relationship with the 

 recent forms be pushed, P. bassi would also appear to be an 

 intermediate form, and this taken with the P. tertiaria charac- 

 ters, would tend to invalidate Fischer's divisions of the recent 

 forms. On the other hand much closer relationship can be made 

 out with Jurassic and Cretaceous forms for botli our fossil species, 

 and this is in direct accord with the position of most other 

 Eocene species in other parts of the world, and may perhaps be 

 taken as a small additional piece of evidence in favour of the 

 Eocene age of the deposits containing them. 



P. tertiaria McCoy may proljably belong to Leptomaria, but 

 P. bassi certainly does not, and shows rather more affinity with 

 Jurassic forms. 



Special interest attaches to this genus as a " persistent type," 

 and on account of its rarity living at the piesent time, and fossil 

 in Tertiary deposits, as compared with its numerous fossil repre- 

 sentatives from older geological deposits ranging up from 

 Silurian. 



There are five recent species of which there are only about 

 twenty-three or twenty-four specimens known. 



1856. Pleurotomaria quoyana, Fischer and Bernardi. 

 1861. Pleurotomaria adansoniana, Crosse and Fischer. 

 1877. Pleurotomaria beyrichi, Hilgendorf. 

 1879. Pleurotomaria rumphii, Schepman. 

 1899. Pleurotomaria salmiana, Rolle. 

 Including the present new species, P. bassi, the number of 

 Tertiary species recorded is twenty, but one at least of these is 

 unknown by any figure or description and ought hardly to be 

 taken into consideration. The remaining nineteen are all very 

 rare, and the majority are recorded from Eocene beds. Altei-ing 

 the age ascribed to P. tertiaria, McCoy, from Miocene to Eocene, 

 the species are distributed as follows :■ — 



Eocene - - 14 species. 

 Miocene - - 2 species. 

 Pleistocene - 3 species. 



