2 PHYLOGENY OF FUSUS AND ITS ALLIES. 



which all the members (species and varieties) are descended from 

 each other or from a common ancestor within the genus. From a 

 study of the Fusidas it appears that the early stages of the shell, i. c, 

 the protoconch and nepionic stages of the conch, are of the greatest 

 importance in this group, as giving evidences of genetic relationship. 

 Parallelism constantly crops out in gastropods, where it appears to 

 be more frequent than in any other group of mollusks. But paral- 

 lelism is no guide to affinity, and hence grave mistakes in classification 

 are made, unless this fact is borne in mind. Parallelism is much 

 more patent in the later stages of development than in the earlier ones, 

 although it is by no means unknown in these latter. 



Two species of gastropods in which the adult characters are 

 similar, but the early characters dissimilar, are undoubtedly less closely 

 related than species in which the early stages are similar or identical, 

 and the later stages diverse. A good example of this is found in 

 the so-called species of Fnsus which are found in the Eocene deposits 

 of the Gulf states. These appear to be typical Fusi as far as the 

 adult form is concerned. Even the early stages {i. e., the nepionic and 

 neanic) are in some cases not unlike those of true Fusi. But the 

 protoconch in one group is like that of Levifiisus or Pleiirotoma 

 {Falsifusus), and in another group it is like that of Fulgur, the 

 nepionic stages in this latter case also being Fulguroid (Fulgurofusus) . 

 It is obvious that, in spite of the similarity of adult characteristics, 

 these species can not be referred to Fnsus, since the protoconch indi- 

 cates that they belong to distinct phyletic series, which are more closely 

 related to other genera than they are to Fnsus (see pi. XVII, figs. 

 5-9, pi. XVIII, fig. I). 



One of the most striking examples of error in classification from 

 want of consideration of the early stages of development is found in 

 the recent genus Cyrtnlus, which by all later authors is united with 

 the Eocene genus Clavilithcs, Cyrtulns being made a synonym. The 

 adult features of the only known species of Cyrtuhis are almost pre- 

 cisely like those of Clavilithcs parisicnsis (Mayer-Eymer), the type 

 of the genus Clavilithcs. This is due to the fact that both species 

 belong to phylogerontic series in which gerontism is expressed in the 

 manner most characteristic of gastropods ; namely, in the loss of orna- 

 mentation as well as the loss of the characteristic form, the successive 

 whorls becoming wrapped about the preceding one in such a manner 

 as to obliterate the original outlines, except those of the spire (see figs. 

 13 and 14, p. 97). This is a manner of growth eminently characteristic 

 of Melongcna, and it may well be denominated a Melongenoid type 

 of growth. Such a manner of growth appears in the terminal mem- 

 bers of a great many phyletic series of gastropods, and the uniting of 

 all such phylogerontic terminals into a single genus or even family, as 



