THE GASTROPODA 121 



wonderfully preserved fossils from the Middle Cambrian 

 of British Columbia, Mr. Walcott has found a Hyolithes 

 showing swimming organs closely resembling those of 

 modern pteropods. It is impossible that pteropods can 

 have existed before the opisthobranchs from which they 

 are derived, and no opisthobranch is known before the 

 Carboniferous period. It is also very unlikely that any 

 group should have abounded in the Palaeozoic era, have 

 lain hidden through the Mesozoic, and have again 

 become abundant in the Cainozoic. The most probable 

 explanation is that we have here again a case of homoeo- 

 morphy, due perhaps to adaptation to like conditions. 

 These Palaeozoic forms should therefore be removed 

 from the Pteropoda and placed provisionally in a separate 

 order for which the name Conularida may be used. It 

 is quite likely that this is not a natural group : some of 

 its members may possibly have been tube-secreting 

 annelids. 



SUB-CLASS: PULMONATA. 



These are terrestrial or freshwater forms, with shells 

 of very variable shape, but always thin and unorna- 

 mented (except by colour, which is sometimes preserved 

 in fossils). Planorbishas been described; other fresh- water 

 forms are Limnaa, with long pointed few-whorled spire and 

 oval aperture (Fig. 35, h), and Physa, of somewhat similar 

 shape, but sinistral. Among land-snails are Helix, the 

 common snail, Pupa, which has a cylindrical form, the 

 maximum diameter being attained very early, and 

 Glandina, resembling Limnaa in form. 



All these genera are common in the numerous fresh- 

 water beds of the Tertiary. Previous to that freshwater 

 deposits are scanty. The oldest, in the Devonian, have 

 yielded no gastropods, though they contain freshwater 

 lamellibranchs ; and in the Coal Measure no freshwater 



