232 



In shape it resembles Melania, with which genus it has been 

 commonly confounded ; but the characteristic A-aricose whorls and 

 the phenomena of the aperture indicate closer affinities with the 

 Cekithiid^, in which family the late Dr. Paul Fischer' and 

 M. Cossmann^ have classified it. 



Type. — Melania costellata, Lamarck. 



Diastema provisi, Tate. 



1894. Diastoma provisi, Tate, Jouru. Eoy. Soc. N-S-W. vol. xxvii. p. 177, 

 pi. X. fig. 6. 



Shell solid, turriculate ; whorls about ten in number, excluding 

 protoconch (which is not preserved in the specimens in the Museum 

 collection), the fine longitudinal and spiral ornament is described 

 in detail by its author ; suture cut in ; aperture oblique, ovate, 

 angular behind, and slightly depressed in front ; outer margin 

 thin ; inner margin callous, a thick, obtuse ridge bordering the 

 posterior channel; columella with a single plication or fold, 

 accentuated in the adult, the attendant callosity spreading over 

 the anterior portion up to the sinuosity in the front of the aperture. 



The costfe of the whorls are more slender and numerous than 

 in the living D. melanoides, Reeve, and the callosity spreading 

 behind the front of the columella is more sharply defined than in 

 the recent shell, with which it presents many striking points of 

 resemblance. Compared with D. costellatum, Lamarck, of the Hamp- 

 shire and Paris basins, amongst other things it is more solid, 

 broader, has less convex whorls, the longitudinal ribs are smaller 

 and closer together, whilst the large ridge and thick callous inner 

 margin are highly characteristic. 



Dimensions. — Length 45 mm. ; breadth 14*5 mm. ; length of 

 aperture 14 mm. 



Form, and Loc. — Older Pliocene : near Adelaide, South Australia. 



G. 9479. Examples illustrating stages of growth. Purchased. 



1 Manuel de Conchyl. 1881, pp. 682-3. 



2 Ann. Soc. Eoy. Malac. Belg. t. xsiv. 1889, p. 30. 



