14 PHYLOGENY OF FUSUS AND ITS ALLIES. 



together. When one of the spirals becomes stronger on the center of 

 the whorl, it generally leaves three, rarely four, and more rarely still 

 two spirals on the shoulder above it. Assuming that the strengthened 

 spiral is in all cases the same one in the same species (presumably the 

 first spiral to appear, if we could trace them back to their beginning), 

 the suggestion presents itself that those shells of the same species, in 

 which only two spirals lie between the strengthened spiral and the 

 suture in the adult individual are more primitive than those in which 

 three appear, and both are more primitive than those in which four 

 appear. This is suggested by the fact that in the shell with four 

 spirals above the carina (i. e., the central stronger spiral) in the adult, 

 there are only three in an earlier stage, above this same spiral, whether 

 strengthened or not, and in a still earlier stage only two. If we go 

 back far enough, only a single spiral would probably appear between 

 that which is strengthened later on, and the upper suture. 



The shoulder above the strengthened spiral or carina is always 

 convex in this species, and never becomes flattened or in any other 

 way accentuated, except by the appearance of the carina. The ribs 

 extend uniformly from suture to suture, diminishing but slightly as 

 they approach the upper one. 



Another modification of the primitive type on specimens which 

 are included within the species is found in the accentuation of the 

 spirals upon the ribs, thus producing a subnodose or subspinulose inter- 

 section. This is a feature eminently characteristic of specimens of this 

 species in the Paris Basin, from which the type of Lamarck was de- 

 rived. It also suggests the British F. asper, in which this feature is 

 strongly developed. 



Below the center of the whorl, and just below the suture, is a 

 broader interspace than elsewhere on most adult specimens of this 

 species. This space, lying between two primary spirals, has in a few 

 of the more accelerated individuals an intercalated secondary spiral 

 in the ephebic stage. This feature might occur in an unaccelerated 

 but extremely long lived and vigorous individual, after the normal 

 adult stage is passed. In a single specimen intercalated spirals have 

 been observed on other parts of the whorls, but these intercalations 

 disappear again, before the shell has reached the adult stage. 



Localities: Barton cliff (M. C. Z. 1401, 1403) ; Hordle cUff (M. 

 C. Z. 1407, 27737) ; London clay, no loc. (M. C. Z. 1406) ; Muddiford 

 Harts (M. C. Z. 27736). 



Horizon: Eocene. 



Two shells from Muddiford Harts are the most accelerated indi- 

 viduals of this species which have come to my notice. They have four 

 and five spirals respectively on the shoulder above the carina, while 

 the more advanced of the two has two intercalated spirals in the broad 



