I08 PHYLOGENY OF FUSUS AND ITS ALLIES. 



two and three whorls of this type exist, and from one to two whorls 

 of each of the preceding. In some specimens the ribbed rugosus 

 stage is scarcely developed. 



All the changes are gradual and the stages merge into each other. 

 The variations of this species lie between C. damcriacensis and C. 

 parisiensis, and the varieties approach the one or the other, according 

 to the smaller or greater amount of acceleration which they have 

 experienced. The greatest variation is in the length of the ribbed 

 portion of the spire, some specimens of this species approaching the 

 more accelerated specimens of C. dameriaccnsis and establishing a com- 

 plete gradation between the two species. 



The concavity seen on the whorls of C. damcriacensis is also slightly 

 traceable on the adult whorls of some specimens of C. conjunctus 

 (Plate XI, fig. i). It usually contains faint spirals while the re- 

 mainder of the whorl is smooth. The concavity is also indicated on 

 Deshayes' figure. 



The neanic stage of this species (Plate XI, fig. 3) is the structural 

 equivalent of the ephebic stage of C. dameriacensis, the shell in both 

 cases being characterized by the absence of ribs, and by well-developed 

 spirals. The conjunctus characters may be assumed long before the 

 shell has reached full size. In that case the young C. conjunctus is 

 indistinguishable from the young C. parisiensis which passes through 

 a conjunctus stage before it attains the parisiensis character of the 

 adult. We may, however, assume with good reason that young shells, 

 in which the rugosus stage persists long and is succeeded by a 

 dameriaccnsis stage, will probably not pass beyond the conjunctus stage, 

 i. e., that these are immature C. conjunctus. On the other hand those 

 shells in which the early stages are abbreviated or eliminated, will 

 probably pass beyond the conjunctus, and into the parisiensis stage ; 

 these, therefore, are the young (neanic) of C. parisiensis. No sutural 

 shelf or terrace appears on this species, though it may appear in an 

 extremely gerontic individual. Nevertheless, in some adult specimens 

 there is a slight thickening below the suture, producing a subterraciform 

 appearance. This is precisely the character of the whorls of an im- 

 mature C. parisiensis. The adult C. conjunctus, therefore, is the struc- 

 tural equivalent of the neanic C. parisiensis. 



Each stage is characterized only by the features which belong to 

 the adult whorls of the species after which the stage is named. Thus 

 the damcriacensis stage is that portion of the shell in which the whorls 

 are round without ribs and marked by spirals. The species (dam- 

 eriacensis) must, however, have these characters in the adult whorls 

 and it must have a rugosus stage preceding it. This rugosus stage 

 may be long or short — in the typical specimens it is long. We may 

 find, however, specimens in which acceleration is confined to the early 



