218 PROCEEDINGS OF THE ACADEMY OF [-^lay, 



If we examine the recent species of Py7'ida,\xe find here again different 

 apical characters which can visually be duplicated in the fossil forms of 

 the American rocks. For instance, though their adult sculptures may 

 differ, the recent species P. decussata and P. dussumieri and the Oligo- 

 cene (or Miocene) P. pilshryi all possess a very similar style of apex. 



At the present day the genus is widely distributed in the shallow 

 waters of the tropical seas. Though these seas are now separated by 

 land barriers, it is probable that in Eocene, Oligocene and Miocene 

 times the marine connections between the different provinces were 

 much more prominent features of the geography. Then, in all likeli- 

 hood, the genus had a continuous distribution around the world; and 

 even where land barriers were interposed near the equator, it is probable 

 that a more generally moderate climate enabled its representatives to 

 pass through latitudes where now their existence would be impossible. 



The differences between Pyrula dussumieri of China and P. decus- 

 sata of the west coast of Central America were probably produced 

 by some change which forbade the intermingling of the inhabitants 

 of the two provinces. At present the deep water of the tropical Pacific 

 and the colder climate in the shallower parts of that ocean are both 

 sufficient barriers to keep the species apart. Wliether their separation 

 w^as originally caused by a change of climate in higher latitudes, or b}' 

 a depression in some part of the bottom of the tropical Pacific, or by 

 the working of both of these factors, it is, of course, impossible to say. 



The relations of Pyrula decussata with P. papyratia are probably 

 more certain. In the Oligocene and Miocene the Pyrulas with the 

 generalized apex had a free passage between the Atlantic and Pacific 

 basins, and there was no differentiation into two geographical provinces. 

 With the close of the Miocene the land barrier of the Isthmus of Panama 

 was thrown up, the two oceans were separated and two geographical 

 provinces created. After this the forms of the eastern waters, for 

 some unknown reason, followed out a line of evolution which w^as chief!}" 

 marked by apical modifications, while the Pacific forms changed 

 slightly in the cancellated stage but retained the old Miocene type of 

 apex. 



Perhaps the most important points emphasized by this study are the 

 following: (1) We may have living at any one time on the earth's 

 surface an assemblage of closely related gastropod species which differ 

 to a marked degree in the evolutional grades which they have attained. 



(2) In a group of closely related gastropod species the chief modifica- 

 tions which are introduced with the passage of time may occur iJiainly in 

 the features of the early ivhorls, while the later adult ivhorls inay re^nain 

 relatively unchanged. 



