17 



The species of the genus Thais arc rather exceptional among 

 lirtoral marine molluscs in developing such numerous and dis- 

 tinctive local races, since the latter as a rule produce active 

 free-swimming larval forms which may be carried great dis- 

 tances, both by their own powers of locomotion as well as by 

 the movement of ocean currents. The only barriers to the 

 spread and intermingling of populations of such species is in 

 the presence of interposing land masses, the direction and force 

 of ocean currents, contact with bodies of water with unfavor- 

 able salinities and temperatures, and the possible transport 

 of the larval forms to areas with unfavorable ecological con- 

 ditions in the littoral zone. The more distant segments of widely 

 distributed forms of this type may diverge to form with the 

 intermediate populations what would appear to be an exten- 

 sive clinal system rather than a rassenkreis. 



Along the borders of the continents areas may be isolated 

 through the development of ecological conditions which arc 

 unfavorable for the majority of littoral molluscs. The most 

 common of these is represented by embayments which receive 

 large volumes of fresh water from inflowing rivers. In such 

 areas there is a tendency to develop local races among the 

 invertebrate inhabitants. The faunas of islands separated from 

 the mainland by various distances would be another example 

 in the same category. 



The writer is inclined to believe that when the taxonomy of 

 many of the groups of marine gastropods is revised to bring 

 them in line with the procedure followed in recent years with 

 the fresh-water representatives of this group, the list of trivi- 

 al names will be greatly reduced, and many forms listed as 

 species, subspecies and varieties, will be given a different and 

 more logical interpretation. 



Most of the attempts to explain the origin of the local races 

 of Thais have been based upon observations made upon the 

 Atlantic species T. lapillus, and the majority of the authors 

 appear to have reached the conclusion that the observed differ- 

 ences are due to environmental factors, either physiological 

 or selective. 



