gibbons: on rhytida caffra. 95 



the species which has suffered least change — account for their 

 absence among the more advanced typical forms. Again, may not 

 the darker shade of color, which often prevails between the zone 

 and the umbilicus of all varieties of B. acutus, be such a vestige 

 diffused into a monochrome ? If not, how is this latter to be 

 accounted for ? 



As this paper is merely suggestive it is hardly necessary to 

 tabulate the undersides, but a series exhibiting the most striking 

 phases should find a place in collections intended like our 

 museums for educational purposes. Series exhibiting variations 

 of size and shape should also not be omitted since the latter, 

 more especially, may have a most important bearing upon the 

 origin of species. 



ON RHYTIDA CAFFRA Fer. 

 -By J. S. GIBBONS, M.B. 



Shell ovato-orbicular, umbilicate, coarsely rugoso-striate; 

 whorls 3 1 to 4, last twice breadth of spire, latter flattened; 

 epidermis thick, reflected over edge of peristome, dark greenish- 

 yellow, transversely streaked with olive-green; interior lined with 

 a thin milky-white membrane. 



Length 1.18; breadth 1.87. 



Animal has the body elevated in front with a broad, rounded 

 dorsum; sides deeply concave; margin of foot slightly spreading; 

 behind the posterior part of the body is broadly lanceolate, 

 tapering to a sharp end, depressed, but bearing a distinct keel; 

 color a dull opaque-grey on the sides — dorsum a rather rich 

 yellowish-umber color, with darker brown along the tail behind 

 and with three broad streaks in front; one mesial, the other 

 somewhat lateral and running from the collar to near the oral 



