PHYLOGENY OF FUSUS AND ITS ALLIES, 3 



is often done, is an i<;-norin,c^ of the principles of genetic classification. 

 Such fusion of terminals of distinct phylctic branches is responsible 

 for the fact that our larsjest j^enera, such as Piisiis, Murex, Plcurotoma 

 and others are overrun with species of wholly foreign ancestry. 



It must, however, be emphasized that the protoconch is not to be 

 alone relied upon for the determination of generic boundaries. A 

 gastropod shell with a Fusoid protoconch is not necessarily a Fusus, 

 for it may have passed beyond the Fiisiis stage, as in Cyrtulus, or be a 

 lateral branch, as in Aptyxis, Rhopalithcs, or Hemifusus. The form 

 and other characters of the adult shell must not differ too widely 

 from those of typical Fusi. An example of similarity of protoconch 

 and early whorls between two distinct, though perhaps related, genera, 

 which is to be explained as a case of parallelism, is found in Clavilithes, 

 and certain species of TnrbincUa of the American Eocene, as noted 

 below. 



No principles have thrown such light on the relationship of mem- 

 bers within a phyletic group, and on the method of their development, 

 as the law of acceleration annunciated by Hyatt and Cope, and the 

 complementary law of retardation of Cope. By an application of 

 these laws to genetic classification, many complex problems are solved, 

 and light is thrown on the relations of the varieties and species to each 

 other, and their position in the phyletic series. 



Modification through acceleration in development may be con- 

 sidered as acting in two ways : first, by condensation, and, second, 

 by elimination of stages. Condensation of stages usually precedes 

 elimination of stages; the stage in question occupying less and less- 

 space in the development of the individual and finally disappearing 

 altogether. Elimination may, however, occur without previous con- 

 densation ; such abrupt dropping of stages indicating a high degree 

 of acceleration. All stages may be equally condensed or some may 

 suffer condensation more than others. Again, condensation may 

 affect only certain structural characters of the shell, while others suffer 

 little or no condensation. Thus in certain species of Clavilithes the 

 shelfless suture is condensed; making room for the characteristic shelf, 

 while some of the other characters, such as the round whorl and spirals, 

 still persist. In other words, some of the later acquired, more special- 

 ized characteristics are pushed back into earlier stages, occurring side 

 by side with characters of a more primitive type. 



The protoconch stage in the Fusidae appears to be the least affected 

 of all the stages by the modifying forces. Condensation appears to 

 a less extent in this stage than in any other. In fact, in some of the 

 specialized Fusidae an elongation of the protoconch is a marked feature. 

 On the other hand, the protoconch of Fusus is generally ornamented 

 by characters wdiich in a more primitive type are found only in the 



