96 THE LIFE OF THE MOLLUSCA 



proportionately nearer and nearer the umbo (Plate 

 XXIX., Fig. 15). 



Another interesting instance of change in growth 

 is afforded in the fossil V elates (Plate XXXII., 

 Figs. 1-6). This, when young, closely resembles a 

 Theodoxis ( = Neritina) in appearance, and, like it, grew 

 spirally. When about half an inch in diameter (i.e., 

 when about four and a half whorls have been com- 

 pleted), however, it ceased to grow spirally, and 

 continued the lip of the aperture in an oblique outward 

 direction, depositing at the same time copious shelly 

 material on the columellar wall of the body-whorl, 

 while it obtained the extra room required within by 

 reabsorbing the inner side of the shell at that point. 

 In the end the adult animal is seen to have occupied 

 a single chamber excavated in previously deposited 

 shell — a unique example of most uneconomical 

 molluscan architecture. 



In most embryo Gastropods an operculum is 

 present, though subsequently frequently discarded 

 (Plate XXIX., Fig. 5, 0). The Pulmonata (except 

 the Auriculidse, Siphonariidae, and Oncidiidae, which 

 have an operculum during development, and Amphi- 

 bola, which retains it throughout life) do not develop 

 an operculum ; furthermore, the veliger stage is 

 reduced or altogether wanting in the members of 

 this order, as it is in viviparous species whose young 

 are hatched in the adult condition. In those Gastro- 



