niYr.OGENY OF FUSUS AND ITS ALLIES. I49 



mergence of the isthmus, and that we may therefore look for Miocene or 

 carHer Tertiary ancestors of this species in the deposits of that age 

 in tropical America. F. gahhi, though suggestive, is not conclusive, 

 as the early stages of this species are unknown, and hence its relation- 

 ship undetermined. F. dupctit-thouarsii var. nodostis is the most 

 primitive representative of this series and from it all the other varieties 

 were derived, as has been shown. F. amhustus, a west coast species, 

 appears to be a lateral descendant from F. dupetit-thouarsii nodosns. 

 Var. irregularis and F. meyeri, clearly derived from the more advanced 

 members of the regular series of F. dupetit-thouarsii^ are probably 

 also west coast shells, though in collections the former has been labeled 

 as coming from^ the East Indies. Considerable doubt is to be en- 

 tertained as to the correctness of this locality, as the specimens were 

 identified with F. longirostris, which it at home in the East Indian 

 waters, and the habitat of which, together with its name, appears to 

 have been arbitrarily applied to the specimens under discussion. 



Having now traced the distributions of Fusus as far as the species 

 have been studied, we may next inquire as to the probable method 

 of migration of these organisms. Was it accomplished along a former 

 continental platform, or was it across an Atlantic and Pacific like that 

 of the present day? From what is known of the habitat of Fusus 

 and its congeners, migration across the oceans on the present ocean 

 bottom is out of the question, for all modern species occur only within 

 moderate depths, being at home only in the littoral district.* 



It is furthermore impossible, that migration should have taken 

 place either along a north or a south Atlantic or Pacific shore line 

 or continental shelf, unless wholly different climatic conditions existed 

 at the time of such migration, for no true Fusi are known to exist 

 outside of tropical or semitropical regions. Even if such conditions 

 may have existed in the north or south Atlantic or Pacific during early 

 Tertiary times, we have no evidence that they obtained in the modem 

 period during which some of the most puzzling transoceanic migra- 

 tions have taken place. 



There seems thus no way to account for the migration of these 

 organisms except by flotation during the larval period of their develop- 

 ment. Nothing is known, so far as I am aware, of the early stages 

 of true Fusus. Whether it has a free meroplanktonic veliger stage, 

 or whether as is the case in closely related types, especially Fulgur 

 and Sycotypus, this stage is passed through within the egg capsule, 

 has still, I believe, to be determined. If the free veliger stage exists, 



* The author follows Ortmann in the definition of the term littoral, including 

 in it the "Flach See," or all that portion of the sea bottom which is effectively 

 illuminated by the sun's rays. Though variable, the limit of this depth is in 

 the vicinity of the hundred-fathom line. 



