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 Velates (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.c., 
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 Auriculidz, Siphonariidz, and Oncidiide, which 
have an operculum during development, and Amph- 
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- 
