CHAP, xxii.] MOLLUSCA. 539 



existed from the earliest Palaeozoic times down to the present 

 day ; while Terebratula, Rhynchonella, Discina, Nautilus, Natica, 

 Pleurotomaria, Patella, Dentalium, 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. 



