86 PLEURODONTE. 



lied by a strong bond of affinity, and well distinguished from all 

 other recent genera. 



The genus stands isolated in the New World fauna, its relation- 

 ships being decidedly with the groups of China, the East Indies, 

 Papua and Australia. Its advent in Middle America is one of the 

 most interesting problems in Helix distribution. The solution of 

 this mystery is not lightened by the known distribution of Glandina, 

 Clausilia, etc., in both the Old and the New Worlds, for no shells 

 in the least allied to this genus of large Helices have been found in 

 European tertiary strata. 



A thorough study of the nomenclature of this group reveals the 

 necessity of several unwelcome but apparently inevitable changes. 

 The well known generic name Caracohis, must be replaced byP/ew- 

 rodonte, which was proposed and defined in a perfectly proper and 

 regular manner by Fischer de Waldheim. It is impossible to use 

 the anonymous, undefined name Lucerna, of Humphrey's sale cat- 

 alogue Museum Calonnianum, unless we disregard the universally 

 recognized canons of nomenclature. 



Subdivisions. 



Pleurodonte may be divided primarily into two subgenera, each 

 of which is split into several minor groups or sections. The latter are 

 practically impossible to recognizably define in words, although not 

 difficult to learn by sight. It will readily be understood, therefore, 

 that no great importance attaches to these lesser groups. They are 

 the natural result ~of late geological changes in the West Indies* 

 which broke the parent stock into island colonies. The whole 

 series tells clearly of a former period of greater elevation of the 

 Antillean region, and closer connection with the middle American 

 mainland. The fact that all of the main modifications are found 

 upon the greater Antilles would lead us to believe that here the group 

 first-became established ; that the Caribbees were peopled from the 

 northwest, and the mainland of South America also from the north ; 

 and that the sections grouped below under subgenus Polydontes are 

 a later modification of the stock, which took place subsequent to the 

 migration to the southward. The full understanding of the distri- 

 bution of these Helices, awaits the explanation by geologists of the 

 main orographic changes of the West Indies during tertiary time 

 an inquiry beset with difficulty, and as yet but little understood. 





