lO PHYLOGENY OF FUSUS AND ITS ALLIES. 



We know so far of only a few genera which show greater accelera- 

 tion in the protoconch than is shown by Ftisns. One of these is the 

 Fasciolarioid genus Heilprinia, of which Fiisiis caloosccnsis Heilprin, 

 is the type. In this the ornamentation extends to within a very short 

 distance of the apex, which is minute. (Plate XVIII, fig. 5.) In this 

 genus, however, no definite line is drawn between protoconch and 

 conch. Falsifusiis (?) apicalis has another remarkably accelerated 

 protoconch which, unlike that of Heilprinia, terminates abruptly. The 

 apex of this species, however, is smooth as in the typical members of 

 the genus Falsifusiis (see pi. XVIII, fig. 2). 



The Conch of Fusus. 



In all the normal species of Fusus the nepionic shell is at first 

 round-whorled, round-ribbed, and furnished with simple uniform re- 

 volving stride or spirals. In a few highly accelerated species, this 

 type of nepionic whorl is crowded out, its place being taken by a later 

 angular-whorled type. Nearly always, however, in such cases, the 

 characteristic round whorl is indicated in that portion of the conch 

 immediately succeeding the protoconch, even though it may, and often 

 does comprise only the merest fraction of a volution. 



The angulation of the whorls is brought about by the relative ex- 

 cessive growth of one or more of the central spirals of the whorls. At 

 first the three central ones are about equally strong and they may re- 

 tain this subequality for a number of volutions. Sometimes the lower 

 spiral is covered by the upper edge of the succeeding volution, in which 

 case the appearance of a bicarinate central portion is given. A true 

 bicarination is due to the suppression of one of the three central 

 spirals, in favor of the other two {Fitsus dupetit-thouarsii). Event- 

 ually, however, in nearly all cases, the multicarinate aspect gives way 

 to a unicarinate one, in which one spiral, usually the central one, 

 becomes stronger than the others. In Fusus toreumus, and some other 

 species, however, the multicarinate feature is retained in the adult. 

 The ribs coincidently fade away towards the sutures ;they remain strong 

 only on the angulation where they commonly give rise to nodulations, 

 which may be rounded or vertically flattened in various degrees, even 

 to pointedness. This condition may continue to the end, or it may 

 become further modified by the total disappearance of the ribs, and 

 with them the nodules. Finally the angulation disappears, the whorl 

 thus again becoming rounded, though now without ribs. The ribs 

 occasionally reappear in the final stages. The spirals remain simple 

 only in the most primitive species ; in all others they are fortified by 

 intercalations at a varying age, according to the species, of secondary 

 spirals. Often tertiary and higher series occur. Occasionally in 

 certain species a strong spiral may become reinforced, by the ap- 

 pearance of fine revolving lines on its side. 



