98 THE LIFE OF THE MOLLUSCA 
In some other Gastropods the final whorls become 
more or less uncoiled as in Diaphora tuba (Testa- 
cellidz), Brachypodella Brooksiana (Plate XXXII, 
Fig. 10) belonging to the Urocoptide, in Hypselos- 
toma, and other Land Snails, as well as among 
marine species such as Vermetus (Plate X., Fig. 17) 
and Tenagodes (Plate V., Fig. 8); whilst in Blzospira 
echinus the whole shell is scalariform. Then, again, 
the aperture may be turned upwards towards the 
spire, as in Anostoma (Plate XIII., Fig. 31) and 
Hypselostoma ; while the last whorl may be carried 
right up and over the spire, as in certain species of 
Opisthostoma (Plate XXXII., Fig. 7). In Gibbus the 
body whorl is flattened and curiously constricted at 
one point, as if the shell had been made soft and 
then pinched up (Plate XXXIL., Fig. 8). 
Some Bivalves continue to increase the thickness 
of their shells by additional deposition to the inner 
layer long after they have ceased to grow outwardly. 
This is especially noticeable in the Oysters, Gryphea 
arcuata (Plate XXX., Fig. 5), Spondylus, and other 
attached forms. In the case of Ostrea cornucopia and 
Spondylus varius (Plate XXX., Fig. 6) the added 
shelly matter is not solid, but full of cavities filled 
with fluid. Pholadidea, a close relative of the Pid- 
dock, or Pholas, fills up the anterior opening of the 
valves with a callous plate. The Ship-worm (Teredo) 
and some other borers are said to close the end of 
their burrows when adult. In Nautilus the last 
