CHAP. XXII.] 
MOLLUSCA. 
539 
existed from the earliest Palaeozoic times down to the present 
day 5 while Terebratula, Uhynchonella, Piscina, Nautilus, Natica, 
Pleurotomaria, Patella, JJentalium, Mytilus and many other 
living forms, range back to the Palaeozoic epoch. That groups 
of such immense antiquity, and having power to resist such vast 
changes of external conditions as they must have been subject 
to, should now be widely distributed, is no more than might 
reasonably be expected. It is only in the case of sub-genera 
and species, that we can expect the influence of recent geological 
or climatal changes to be manifest ; and it must.be left to special 
students to work out the details of their distribution, with 
reference to the general principles found to obtain among the 
more highly organised animals. 
