PIIYLOGENY OF FUSUS AND ITS ALLIES. I5I 



derived from an Eocene Fusus, nevertheless, belongs to an entirely 

 distinct branch. As has been shown, there are constant differences 

 between the protoconchs and young conch of the American and 

 Parisian Clavilithoids, and these differences appear to be due to genetic 

 distinctness. 



I believe that the Lower Eocene Clavcllofiisus is the phylo- 

 gerontic derivative of an Eocene Fustis, just as the modern Cyrtulus 

 is the phylogerontic derivative of a modern Fnsus. It is not improb- 

 able that Clavilithes, essentially a middle Eocene genus, was derived 

 from Clat'ellofiisits, though this point is by no means clearly deter- 

 mined. In fact, Clavilithes riigosns, the radical of this series, in all 

 but the elongated protoconch, approaches Fitsus and may have been 

 independently derived from that genus. The derivation of the various 

 species of Clavilithes of the Parisian Eocene from the radicle C. 

 rugosiis has been traced, the series being a remarkably complete one. 



Nothing so much argues for the provincial character of the Paris 

 Basin fauna than the distinctiveness of the species of Clavilithes which 

 it embraces. In the closely adjoining British water no identical 

 forms existed, a marked individuality characterizing all the species. 

 That there was a barrier between the two neighboring localities seems 

 unquestionable, but that barrier was probably not land ; nor was it 

 absolutely insurmountable. Nevertheless, those forms which did 

 transgress the limits of the province within which the series developed, 

 were either specifically modified or soon developed characteristics which 

 pointed to a degeneration. It is highly probable that the barrier was 

 merely caused by change in the facies of the Eocene sea bottom, 

 which change is clearly indicated in the lithic character of the corre- 

 sponding deposits. The north German Oligocene province was less dis- 

 tinct in facies or fauna from that of England, and the intermigration 

 of species was probably more pronounced. 



The Eocene of the American Gulf States had likewise its distinct 

 series of species which paralleled those of the Paris basin. The 

 succession of characteristics in the American as in the French species 

 is such a normal one, and the scries in each case fall so naturally into 

 species marking the successive stages in development that we need not 

 be surprised to find the specific characters identical, though char- 

 acteristics of a higher taxonomic value maintain a constant difference. 

 In other words, the same species marking the same stage in the develop- 

 ment of the series occurs in both genetic groups. In the several Eocene 

 provinces of France two other distinct series of phylogerontic fusoid 

 gastropods originated most likely from a Fiisits radicle. These were 

 Rhopalithes and Cosmolithes. Both have distinct generic characteris- 

 tics. Init in each series, species occur, which parallel those of Cla^'ilithcs. 

 Rhopalithes has a typical Fusus protoconch, and is probably not far 



