632 



THE AMERICAN NATURALIST [Not.. XLI 



members of any number of divergent genetic series. Conversely, 

 however, the want of such accelerated conditions in members of 

 the same genetic series may well l)e regarded as sufficient for 

 generic distinction, though this is largely a matter of personal 

 opinion regarding the elasticity of the generic boundaries. Even 

 greater acceleration than this is shown by many genera. Thus 

 certain species of Semifusus have dropped out the round-whorled 

 ribbed stage, the conch beginning with the angular stage (Studies 

 I, Fig. 5). 



In many cases the early characters appear not to have been wholly 

 dropped, but greatly condensed, so that the protoconch quickly 

 merges into a highly specialized conch, the transitional stages 

 being extremely short and often scarcely recognizable. Then, too, 

 some of the early stages may drop out without the abrupt change 

 seen in Fusus, etc. Thus characters which in the phylogeny of 

 the group were developed only at a relatively late period after 

 other characters had come into existence, may in the specialized 

 members of this series appear immediately after the protoconch, 

 the earlier characters being dropped out of the ontogeny. On the 

 other hand, certain persistent characters may be pushed far back 

 into the ontogenv, and apjicar e\en in the protoconch stage. This 

 is seen in the ril>!<'t.s of the last whorls of many protoconchs (Fusus) 

 and in the a])pearan< (' of an angulation or carina in others (certain 

 Murices, etc.). 



Not only is acceleratif)n l)y condensation and elimination active 

 in the eariiest conch stage, but it is often found at a later period, 

 where some shell character, not strongly fixed in the organization, 

 may be eliminated to make room for a later and more prominent 

 one. This condition has already b^en briefly described for Fulgur 

 and Semifusus (Studies I, p. 932) and more fully for Fulgur in a 

 later paper (Studies II, p. 528). It may be briefly reviewed here. 



In both genera, and in the case of Pugilina in what are com- 

 monly regarded as varieties of the same species, the tubercles are 

 normally developed as a result of the concentration of the ribs 

 upon the shoulder angle. This is characteristic of the earliest 

 Miocene Fnlgnrs as well as the ancestral forms of Semifusus and 

 Pugilina. ^^■ith further development the tubercles grade into a keel 

 and this into a smooth rounded and ribless whorl, differing from the 



