l6 PHYLOGENY OF FUSUS AND ITS ALLIES. 



state, while the absence of ribs indicates the latter. It is hardly ex- 

 pectable that there should be a Fusus primitive enough to be without 

 ribs, for with the highly accelerated ribbed protoconch characteristic 

 of this genus, we should assume that the most primitive Fusus conch 

 is ribbed. Therefore, since ribs disappear in the old age of most Fusi 

 it is most proper to consider this species as a descendant of the ribbed 

 F. porrectns, but one in which the ribs have been almost entirely sup- 

 pressed. The loose coiling is not inconsistent with the gerontic state, 

 while the simple spirals may indicate that the series of which this 

 species constitutes the phylogerontic terminal, is a primitive one. 



Locality: Barton clifif, coll. Duval. (M. C. Z. 1408, 1409). 



Horizon: Eocene, London clay. 



FUSLS ASPER Sowerby. 



(Plate I, figs. 7 and 8.) 

 1821. Fusus asper SowErby, Mineral Conchology, vol. 3, p. 131, pi. 274, figs. 4-7. 

 The protoconch is of the normal type, smooth in its early stages, 

 and with vertical closely crowded riblets in the last half volution. The 

 conch has nearly all the whorls angulated and ornamented with strong 

 spirals. In the young shell two central spirals are stronger than others. 

 Above these, on the flattened shoulder, are two strise, and below them, 



a h 



Fig. 3, a, b. Fusus asper. a, a specimen enlarged ( X |) ; b, the protoconch 

 and early whorls still more enlarged (X Y)- M. C. Z. 1406. 



just above the succeeding whorl, is another spiral which during the 

 later stages becomes nearly as strong as those above it. In these later 

 stages the three spirals are the most prominent on the she'd. Where 

 the spirals cross the ribs, flattened asperations or nodulations are pro- 

 duced, which are especially strongly marked in the adult stage. This 

 species is derived from the British varieties of F. acicidatiis, the strong 

 ribs of F. asper being incipiently developed in that form. The small 

 number of spirals on the shoulder and the simple character of these 



