THE GASTROPODA 105 



having a posterior canal: it therefore means that each 

 posterior canal is actually absorbed during the growth- 

 period that follows its abandonment. This suggests the 

 possibility that in some genera the whole of the special 

 structures at the aperture might be absorbed when 

 growth restarts. These considerations have a bearing 

 upon some features found in Cephalopoda (p. 160). 



It may be mentioned that in one genus of gastropods, 

 Distortrix (very rare as a fossil) the method is adopted of 

 forming an enlarged aperture and afterwards recovering 



AC 

 FIG. 31. TRIVIA AVELLANA (J. SOWERBY), PLIOCENE, (xf.) 



A.C., Anterior canal ; P. C., posterior canal. (Original.) 



the normal whorl-form, not abruptly as in other genera, 

 but gradually : the result is an extraordinary and puzzling 

 distortion of form. 



6. Trivia avellana (Fig. 31), found living in British 

 seas and fossil in the Crag, is a small hemi-ellipsoidal 

 shell in which no spiral coiling is recognizable externally, 

 because the last whorl completely envelops and hides 

 the spire. Such a shell is said to be convolute. The 

 aperture is very narrow, nearly parallel-sided and slightly 

 curved, with slight anterior and posterior canals ; the 

 outer lip is thickened. The whole surface is ornamented 



