g8 THE LIFE OF THE MOLLUSCA 



In some other Gastropods the final whorls become 

 more or less uncoiled as in Diaphora tuba (Testa- 

 cellidse), Brachypodella Brooksiana (Plate XXXII., 

 Fig. 10) belonging to the Urocoptidse, 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 Blceospira 

 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 XXXII., 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, Gryphcea 

 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 



