I'UVLOGKNY OF FUSUS AND I'lS ALLIES. lOQ 



stages ; /. c, tlie nigosits stage may be condensed and the dameriacensis 

 stage remain for a longer time, probably through adult stages. This 

 would still be a C. dameriacensis, but a variety in which the early stages 

 are accelerated. Thus specific characters are based on adult char- 

 acteristics to a large extent ; i. e., on the stage wdiich the adult has 

 reached in (leveloi)ment. If a specimen should be found in which 

 the nii^osiis stage has been dropped altogether and the dameriacensis 

 stage follows on the protoconch, and never reaches the conjunctus stage 

 even in the ephcbic condition, it might be desirable to call it another 

 species, even though the adult has the true characters of F. dameria- 

 censis. It is not likely, however, that acceleration would act so locally 

 for we may expect that the spirals become obsolete and a true conjunctus 

 stage appear. In a highly accelerated C. conjunctus the rugosus stage 

 may be eliminated, but not the dameriacensis stage. On the other hand, 

 acceleration may act not in the nej)ionic but in the neanic stage (Plate 

 XI, fig. 2). The rugosus stage may be well developed, and may 

 have strength to resist condensation so to speak, but the dameriacensis 

 stage may be less resistant, and hence be eliminated by acceleration. 

 Thus the conjunctus stage may follow immediately upon the rugosus, 

 without or with but a short dameriacensis stage between. 



Again, acceleration may act e({ually on all stages, condensing the 

 earlier ones, but not eliminating any of them. Then the typical form 

 of the species is produced. 



Localities: Paris Basin (Cuise) (M. C. Z. 1068) ; Parnes (AI; C. Z. 

 1065, 1066, 2/y4^) ; Chaumont (Acad. Sci. 6882) ; Paris Basin (M. 

 C. Z. 1067, 27764). 



Horizo)i: Middle Eocene, Calcaire Grossier. (Also recorded by 

 Deshayes from the Sables moyens. Upper Eocene.) 



CLAVILITHES CONJUNCTUS, senile variety. 

 (Plate XIII, fig. 10.)' 



This specimen differs from the normal form of the species in its 

 acceleration of senile characters, wdiich in the present individual appear 

 in the late neanic and early ephebic stages. The chief characteristic 

 is the looseness of the coil at these stages, which results in the pro- 

 duction of an external sutural canaliculation. A slight looseness of 

 coiling is observable in some typical young of C. conjunctus. and in 

 the present individual a similar slight looseness goes back to the late 

 dameriacensis stage. The separation increases steadily, until the speci- 

 men at the age and size of the early ephebic of a normal C. conjunctus 

 shows a very decided loosening of the last whorl. Coincident with this 

 feature the last whorl has become more cylindrical, departing from 

 the rounded character normal to C. conjunctus. The apertm-e has also 



