26 PHYLOGENY OF FUSUS AND ITS ALLIES. 



merges into a strong- tuberculated keel. As in F. torcmnns, the ribs 

 become fainter and fainter away from the periphery, until finally the 

 keel alone is characterized by them. Even in the coloring the F. 

 toreumus character is maintained, this consisting of brown spots in the 

 inter-tubercular spaces. This is also true in general of the turriculus 

 stage, in which the brown color is confined to the spaces between the 

 ribs. 



The toreumus stage covers usually from three to four volutions, 

 after which the tubercles become obsolete, and finally disappear. There 

 remains then finally a smooth keel, more or less strongly marked in the 

 center of the whorl, and giving it a subangular appearance. This may 

 be regarded as the coins stage ; but in a typical F. coins, all three stages, 

 viz., the turriculus, toreumus and coins stages are present. There may 

 be a greater or less development of one or the other, according to 

 individual acceleration or retardation, but none is absent. The oc- 

 currence of these three stages clearly establishes the ancestry of this 

 species, and marshals into proper array the other species of this series. 



In many cases a fourth stage occurs, in which the keel becomes 

 suppressed, a smooth rounded whorl remaining. This is the longicau- 

 dus stage, and marks the early stages of gerontism in this species. The 

 appearance of this stage does not make a F. longicandns of this species, 

 since in the typical members of that species the torcnmus stage is sup- 

 pressed. It is simply a case of individual acceleration, so that normal 

 gerontic characteristics appear in the ephebic stage of an otherwise 

 normal F. coins. 



The coins stage — a well-developed median keel — may occupy only 

 a portion of a volution, or it may extend over a volution and a half. 

 The longicaudus stage seldom occupies more than a small part of a 

 volution. Two specimens from the Indian ocean (M. C. Z. 32) show 

 pronounced variations. One is accelerated, showing angulation and 

 intercalated spirals on the second or third volution, and a keeled 

 (coins) stage covering over a volution and a half. This is succeeded 

 by the keelless (longicandns) stage, covering a fraction of a volution. 

 The earlier stages being shortened, room is made for the introduction 

 of additional stages at the end. This is not to be regarded as indi- 

 vidual senescence, but as a case of acceleration in development, there 

 being nothing in the character of the shell to indicate that the indi- 

 vidual was not perfectly vigorous. (Plate II, fig. 8.) 



The other specimen is an example of a retarded individual. It has 

 five shouldered and tuberculated whorls following the turriculns stage. 

 There is no coins stage, the tubercles persisting to the end (Plate 



11, fig. 9)- 



According to the definition of our species, this specimen should be 

 classed with F. toreumus, never passing beyond the toreumus stage. It 



i 



