226 HUGH WATSON. 



which probably necessitates a greater change in the structure 

 of the animal) is for the whorls themselves to become laterally 

 compressed. This has taken place to some extent in Strepto- 

 stele and Obeliscella among the Streptaxidse, but it is 

 especially characteristic of the Oleacinidge. In some 

 members of this family, such as Streptostyla gracilis 

 I*ilsbrij, the whorls have become so narrow that the form 

 of the shell closely resembles that of the marine carnivorous 

 genus Con us — a striking example of convergence due to the 

 acquisition of similar habits. Thirdly, the position of the shell 

 may be altered, so that the part of the animal in front of the 

 shell becomes lengthened. This is admirably shown in Hedley's 

 figure of Rhytida lampra {Pfr.),^ but it occurs to some 

 extent in most of the carnivorous genera. A variety of this 

 modification is fouud in such forms as Ennea densecostu- 

 lata Mlldff. and Diaphora eutrachela Mlldff., in which 

 the greater part of the shell has moved further back by the 

 elongation of the last whorl towards the mouth. Fourthly, 

 the shell may not only be moved backwards, but it may 

 become reduced and flattened, until it either disappears 

 altogether as in Selenochlamys, or sinks into the skin as 

 in Apera. No projection will then remain to impede the 

 animal's progress when it follows worms into their burrows, 

 and it is more especially in the vermivorous genera that this 

 extreme modification seems to have occurred. We see, then, 

 that the acquisition of carnivorous habits may lead to a great 

 alteration in the structure, form, position, or size of the 

 shell. 



The movement of the shell to the hind end of the body, and 

 its gradual degeneration, will greatly modify the animal itself. 

 Many of the organs will be subject to partial detorsion. The 

 lung and respiratory orifice will move back with the shell and 

 mantle, so that the animal will become opisthopneumic ; and 

 it is probable that it is a direct advantage to a carnivorous 

 form to have its respiratory opening near the hind end of the 

 back, because this is the part which is least likely to be 

 ' ' Proc. Linn. Soc. N.S.W.* (2nd ser.), 1891, vol. vi, pi. iii, fig. 3. 



