204 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
at the time when the livers were passing from the thoracic to the 
abdominal position such as a shell would cause. There is reason 
also to believe that this shift took place early in the history of the 
Pagurid group. Finally, the chances that the ancestral hermit crabs 
would use dextral shells rather than other objects at the time when 
they began to seek movable residences, are rendered very great by 
the age of the Gastropod Mollusca as a group, while the Pagurids 
are of comparatively recent origin. 
