HELIX-ACAVUS. 77 



(when fresh) by a thin greenish coat, composed probably of algse, 

 which almost entirely obscures the conspicuous hues of the under- 

 lying shell-substance. Cabinet specimens are generally denuded of 

 this extraneous layer. The two species (waltoni and skinneri) which 

 retain the epidermis in part, do not possess this dull outer coat. 



The axis is hollow, but closed in adult shells by the broadly ex- 

 panded columellar lip. The aperture is very oblique, truncate- 

 oblong ; the lip broadly expanded and revolute ; the entire length 

 of the columellar lip is reflexed and adnate to the base of the shell, 

 spreading into a broad flat or excavated plate. The young shell is 

 subglobose, with rounded periphery. The egg, as in all other 

 members of the subgenus Macroon, is large (about i the diameter 

 of the adult shell). It is calcareous. Sarasin says : In moist places 

 in Ceylon, under the mould around the roots of large trees, one often 

 finds white hard-shelled eggs, the size of a small bird's egg. The 

 people know them well, and enjoy opening them to find the young 

 snail which each contains. The sight is indeed unusual enough to a 

 European ; for the young snail stays, like a chicken in its egg, so 

 long as it has room to grow ; and its shell shows remarkably fresh 

 and varied colors. These are the eggs of H. hremastoma, and allied 

 species such as H. waltoni, everywhere abundant in the southwestern 

 part of the island. 



I have excluded those species from New Guinea which Messrs. E. 

 A. Smith and Tapparone-Canefri refer to Acavus. I have examined 

 several of them and find that the embryonic shell (and consequently 

 the egg) is small. This character at once removes them from the 

 vicinity of Acavus, and shows them to belong to the Papidna ( Geo- 

 trochus] series. Of these excluded species, H. brumeriensis Forbes, 

 is figured on PL 12, figs. 41, 42, 43. H. comriei Angas on PI. 16, 

 figs. 8, 9. H. coraliolabris Smith, PI. 17, fig. 13, and H. latiaxis 

 Smith, PI. 17, fig. 16. The descriptions will be found under the 

 subgenus PAPUINA (Geotrochus of authors). 



Groups of Acavus. 



Group of If. hcemastoma. The shell is conoidal, the spire 

 elevated, its whorls convex. The surface is wholly deprived of 

 cuticle or epidermis, except just back of the lip where it sometimes 

 is preserved ; but the outer surface has, in living or fresh shells, a 

 thin coating of greenish alga?, which hides the bright underlying 

 colors. The post-embryonic growth comprises about 1* whorls. 



