52 A MONOGRAPH OF 



Section B.— O P E R C UL A T A . 



PNETJM02T0P0MA.— P/eiJer. 



Terrestrial Mollusca, breathing air by lungs, and furnished with an 

 operculum attached to the foot of the animal, by which it closes the 

 aperture of its spiral shell. 



All the animals of this Order strongly resemble each other in struc- 

 ture, by which, besides the operculum, they are easily distinguished from 

 the other terrestrial Mollusca — Limacidce, Helicidce, &c. The sexes are 

 strictly distinct, never hermaphrodites, as in the Helices, and the animals 

 bear two contractile (not retractile) tentacles, at whose posterior or ex- 

 terior bases the eyes are placed. In some the mantle margin is free, by 

 which character Ferussac distinguished the Helicidce from the Turbiiies ; 

 in others the mantle is entirely within the shell. 



Sub-Order L— OPISOTHALMIA.— P/eijfer. 



Eyes placed on the upper part of the head, behind the base of the 

 subulate tentacles ; foot rather short. 



Family I.— ACICULAOEA.— P/ei^er. 

 Operculum thin, spiral, few whorled. 



Genus— TRUNCATELLA.—i2mo. 



Shell minute, imperforate, turreted, adult always truncated, sub- 

 cylindrical, smooth or ribbed ; aperture oval or elliptical, entire ; peris- 

 tome continuous, straight or slightly expanded, simple or duplicate ', 

 operculum thin, horny, hardly sub-spiral, nucleus basal. 



Animal. — Tentacles two, short, triangularly diverging, cylindiicalr 

 obtuse ; eyes centrally behind ; foot short, rounded at each end ; head 

 proboscidiformed, bi-lobed, by which and the short foot the animal is 

 enabled to creep. 



Found abundantly on the rocks and sea-weeds between tide-marks, 

 and the dead shells are often washed together in vast quantity. There 

 are seven described Australian species, three of which are found on the 

 islands in Bass Straits. 



1. — Truncatella scalarina — Cox. 



Pro. Zool. Soc, 1867, p. 40. 



Mon., p. 95, pi. XV., jfig. 10, 10a, 106. 



Tenison-Woods, Census Marine Sh., Tas., 1877. 



Shell imperforate, fusifoimJy turreted, smooth, shining, white ; spire 

 decollated ; suture constricted ; whorls 4 at the least, convex, strongly 

 longitudinally and regularly ribbed, last equalling in length the three 



