l'.)()7.J NATUHAL SCIKXCKS OF I'lII LADKI.I'HI A. 217 



grouped together as P. papyratia. When we compare P. pilshryi with 

 P. (kcussata of the Panamic geographical province we find that the 

 Pacific form is distinct in its later whorls, but very similar to the fossils 

 in the characters of its apex. 



Altogether this evidence points to free communication between the 

 Atlantic and Pacific basins during Miocene time, a time when most 

 Pyrulas possessed the primitive type of apex. With the end of the 

 Miocene this strait betw^een the two oceans was closed by the elevation 

 of the land bridge which has ever since connected the continents of 

 North and South America. From the time when this barrier was 

 interposed down to the present the forms of the Caribbean and 

 Gulf regions have undergone a striking evolution, marked by the 

 accleration of the cancellated stage and the enlargement of the initial 

 whorl. On the other hand, the forms inhabiting the waters on the 

 Pacific side of the isthmus have retained the ancient apical features of 

 the Miocene. 



Summary. 



The assemblage of forms treated in this paper is particularly well 

 adapted for showing the mutations and variations of a gastropod stock. 

 In order that the results may be of value it is essential that the group 

 be a restricted one. Pyrula, or at least that part of the genus here 

 considered, fulfills the above conditions. The distinctions between 

 its species are so slight, and they are all so unlike the examples of other 

 genera, that we may well feel that they constitute a single genetic stock. 



In addition to the slight but more apparent specific differences 

 furnished by the cancellated stage we have the differences exhibited 

 by the apices. If we trace the members of the genus back into the 

 Tertiary, we find every gradation between the two extremes of apical 

 modification. 



The more important changes Avhich have taken place since the late 

 Eocene are found not so much in the adult sculpture as in the features 

 of the apex. We have, especially in America, a regular series, beginning 

 with forms having a smooth stage of two or three whorls, followed by 

 species in which the smooth stage is more restricted, and finally termi- 

 nating in Pyrula papyratia of Gulf and Caribbean waters, whose 

 smooth stage does not persist beyond the close of the first whorl. This 

 change from the two- or three-whorled smooth stage to the one-whorled 

 smooth stage is accompanied throughout by the gradual enlargement 

 of the early whorls. The more restricted the smooth stage becomes 

 the larger grows the initial whorl. 



