PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 571 



less exactly parallel in the several series. I have attempted to express 

 the above ideas graphically in the following diagram, in which the least 

 evolved species of each group are placed below, while the carinate forms 

 are placed above in each group. 



Outer lip not varicose. 



Lip varicose. 

 rushii 



philippianus 



[In these diagrams the most primitive, Naticoid species of each group are below ; those modi- 

 fied by the development of carinae above ; and the right-hand column contains derivative species 

 having the outer lip varicose. The connecting lines indicate the chief affinities of the forms.] 



While the above diagrams are not intended for phylogenetic trees, all 

 the species being contemporaneous, yet it is likely that the ancestral forms 

 in groups II, III and IV did not differ materially from the least differ- 

 entiated of the recent species. It is significant that the only form known 

 to have a wide distribution, P. lapidum, is one of the least specialized of 

 the genus. 



The evolution of carinae and varices in the several groups seems to have 

 been homoplastic. It will be shown below that the keels are superposed 



