46 PHYLOGENY OF FUSUS AND ITS ALLIES. 



7. FUSUS DUPETIT-THOUARSII AND ITS ALLIES. 



FUSUS DUPETIT-THOUARSII Kiener. 



(Plate V, figs. 1-5.) 



1842? Fttsus dupetit-thouarsii Kiener, Iconographie, XIV, p. 5, pi. 11. 

 1847. F. dupetit-thouarsii Reeve, Iconica, sp. 9. 



The protoconch of this species is of the normal fusoid type, consist- 

 ing of about one and one-half volutions. The first whorl is smooth, ob- 

 liquely erect and the apex partly covered by the succeeding whorl. The 

 last portion of the protoconch is furnished with narrow smooth vertical 

 ribs, more than their own width apart. On the last part of this ribbed 

 portion of the protoconch two faint spirals appear in the center of the 

 whorl. These spirals of the protoconch appear gradually, there being 

 no line of demarkation between this part of the protoconch and that 

 with simple riblets. No varix occurs at the end of the protoconch, but 

 an abrupt change is noticeable. This is the most accelerated type of 

 protoconch yet observed in Fusus, the appearance of the spirals placing 

 it ahead of the other species of Fusus, except perhaps F. distans, in 

 which very faint spirals appear between the last ribs of the protoconch. 



The conch begins abruptly, with strong, wide and rounded ribs 

 which are close together, the interspace being reduced to a mere de- 

 pressed line. Several additional spirals appear, the two central ones, 

 however, being strongest. As they increase in size, they soon give a 

 bicarinate and subangular aspect to the whorl. This is the most char- 

 acteristic feature of the young shell. Sometimes this bicarinate aspect 

 of the whorls is marked from the beginning, at other times it does not 

 become prominent until the third or fourth whorl. After the ap- 

 pearance of the bicarination the whorls become increasingly angular, 

 the shoulder flatter and the ribs weaker. The bicarinate aspect con- 

 tinues through seven or eight whorls. 



After this stage in the ontogeny is reached a divergence occurs 

 which produces several varieties, which might well be considered dis- 

 tinct species. 



Var. NODOSUS var. nov. 



(Plate V, fig. I.) 



This is the most primiiive variety of this species yet observed. It 

 represents the tubercnlatus (toreumus) stage of this series. The upper 

 of the two central carina becomes stronger, and a more pronounced 

 angulation appears. The ribs become obsolescent on the shoulder and 

 on the body, remaining on the periphery only as nodules. .These 

 nodules continue to the end in the most typical individuals. 



Intercalated spirals appear early, while the whorls are still bicarinate. 

 In the later whorls, when the upper of the central spirals becomes the 

 strongest, the spiral next above also increases in strength, so as to 



