6 University of Michigan 



that these differences were sexual, in a number of instance^ 

 the male and female shells were described as distinct species. 

 This modification of shell in the female is practically the only 

 change, excepting, perhaps, the sculpture of the beaks,* that 

 has taken place, so far as we know, in the structure of the 

 shell itself. 



11. Environmental Factors Affecting Evolution. 



These are primarily migration and isolation, and it is nec- 

 essary to briefly consider what they have been. 



Migration. — The oldest land in eastern North America 

 is w^hat is known as the Laurentian Highlands in eastern 

 Canada. When they first emerged from the sea they were 

 separated from the earliest western land, the Sierra Nevada 

 Mountains, by the sea itself. Later, as the land continued to 

 rise, the southern Appalachians were uplifted and in the first 

 instance, and through Carboniferous times, these, and their 

 western extension, now known as the Cumberland Plateau, 

 extended continuously from the eastern states southwest into 

 Texas. This highland was the original divide between the 

 drainage systems of that time. On the east and south the 

 rivers flowed into the Atlantic and the Gulf of Mexico. North 

 and west of this divide the streams flowed westerly into the 

 Mesozoic Sea, which extended from the Gulf of Mexico to 

 the Arctic Ocean. It was in the swamps, streams and bayous 

 which lay along the eastern shore of this ancient sea that in 

 Cretaceous times were evolved from the primitive forms, com- 

 ing down at least from Triassic times, the prototypes of the 



* There is a very considerable amount of variation in tlie recent fauna in the 

 sculpture of the beaks. These differences are generallv recognized as valid generic 

 distinctions. But we have not as yet accumulated sufficient facts in regard to the 

 development of this feature, either in recent or extinct forms, to formulate any 

 very definite theory as to either the cause, or manneil, of its evolution. The whole 

 subject is still in the speculative stage. 



