166 CRUISE OF THE BARRERA 



ing in time Pulmonates. Although they still retain 

 the normal gills of prosobranch mollusks, they have 

 acquired power of absorbing into the blood the life- 

 giving oxygen from the air, if only their gills are 

 moist, and they pass days and weeks without enter- 

 ing the water. Other marine mollusks that con- 

 spicuously inhabit this same sort of station are the 

 Neritas which seem to be traveling along the same 

 route towards a terrestrial existence. It is more 

 than likely that all the great assemblage of land 

 mollusks have in the remote past a marine an- 

 cestry, and the gradual process of change from 

 gills to a lung sack is the result of just such change 

 of habit as noted in the forms now found upon these 

 rocks. 



About the submerged base of this little cliff we 

 searched for possible rarer forms of lesser general 

 distribution. Such stations are usually very pro- 

 ductive and merit careful exploration. One find 

 of considerable interest among the mollusks was 

 a large black Drillia of uncertain identification. 



The deserted house perched upon the ledge and 

 backed by a cocoanut grove has very high ceilings 

 and is built of hard woods in a manner denoting 

 something more important than the ordinary 



