59 



During the periods when the northern seas were not locked 

 in ice and the disposition of sea and land was favorable, migra- 

 tion was possible across the northern borders of the Eurasian 

 and North American continents. According to Davies (1934) 

 four gastropod genera which had been extensively developed 

 in the North Pacific area, suddenly appear as elements in the 

 fauna of the North Atlantic; these were Buccinum, Neptunea, 

 Searlesia and Thais {Nucella)^ including the stock from which 

 the modern forms of Thais lapillus have been derived. These 

 facts indicate that the Thais fauna ol the North Atlantic is 

 little more than an extension of that found in the North Pa- 

 cific, and explains why the local races of Thais lapillus exhibit 

 a remarkable parallelism with those found in Thais lima and 

 related forms in the Pacific. 



As has been pointed out, Dr. H. Staiger found a striking 

 example of chromosomal polymorphism in populations of 7'^jtJ 

 inhabiting the coast of France, the number of chromosomes 

 ranging from thirteen to eighteen, with corresponding differ- 

 ences in morphologyand ecological adjustments. Whether these 

 or similar conditions exist elsewhere on the coasts of Europe, 

 or on our eastern coast, appears to be unknown. In Thais 

 lamellosa as determined by Staiger, and apparently in our 

 other west coast forms, the chromosome number is thirty-five, 

 which seems to be common to several muricid genera. 



On the whole it would seem wise to use chromosome counts 

 as a means of determining taxonomic relationships with due 

 caution, especially in view of some of the recent work on chro- 

 mosomal polymorphism in plants. Muntzing(i954) in a study 

 of the local races of the grass Poa alpina L., which ranges 

 from Scandinavia to the mountains of Central Europe, found 

 chromosome counts varying from 26 to 61, with a correspond- 

 ing variation in morphology and coloration. 



Although the theory of isostasy may explain the major crust- 

 al movements, other factors are supposed to be involved in 

 modifying the level of the sea. The withdrawal of vast amounts 

 of water from the ocean to build the great continental glaciers 



