I02 PHYLOGENY OF FUSUS AND ITS ALLIES. 



This species corresponds to ClaviUthes tiibcrculosiis Desh., which 

 in that series occupies the same stage in development. 



Localities: The type specimen is from Cuise (M. C. Z. 27729) ; 

 Soissons (M. C. Z. 27732). 



Horizon: Lower Eocene. 



CLAVELLOFUSUS MACROSPIRATUS sp. nov. 



(Type Plate I, fig. 28.) 



(See also Plate I, figs. 21, 24, 25 and 27.) 



This species is the most accelerated of the group, paralleling the 

 French ClaviUthes macrospira Cossmann, or the British C. solandcri 

 Grabau, of which these specimens seem at first sight to be diminutive 

 representatives. The early stages are as in the other species of this 

 genus, but the ribbed whorls are fewer in number. The last of this 

 series of whorls lose their rotund character, the ribs at the same time 

 changing into mere undulations. With the disappearance of the ribs, 

 or even before this, the sutural shelf appears, the whorls at the same 

 time becoming cylindrical. This implies a complete dropping out 

 of the tnhercidatus stage, i. e., the stage characterizing the adult 

 Clavellofnsns tuhercnlatns. This stage generally occurs in unaccel- 

 arated species of this and other series, following the ribbed, and pre- 

 ceding the cylindrical-whorled stage. The present species appears, 

 tnerefore, to be derived from C. spiratus, through a process of accelera- 

 tion by elimination, the tuhercnlatns stage being eliminated. 



The majority of the specimens of the genus which have been seen 

 in the collections belong to this species, the group as a whole being 

 strongly accelerated. 



In fig. 25 of plate I is shown the young of the most accelerated 

 individual of this series which has yet come under my observation. 

 The ribbed stage is well developed, and while the whorls are still 

 marked by coarse ribs, a sutural shelf appears. At first the whorls 

 continue round, but soon they assume the cylindrical form with the 

 sides parallel to the main axis of the shell. The ribs, however, con- 

 tinue, there being about a whorl and a half, which are furnished with a 

 shelf and ribs at the same time. It might be considered that we have 

 here a ribbed tnhcrculatiis stage followed by a ribbed cylindrical or 

 subconic stage. 



This is, therefore, an example of an accelerated type, in which one 

 feature (the shelf) has become excessively accelerated, appearing while 

 some of the other primitive features are still retained. In the latter 

 part of the ribbed and shelved whorls the shelf or terrace even projects 

 slightly, suggesting the scalaris stage of the ClaviUthes series (vide C. 

 scalaris). There is some considerable variation in the number of 

 ribbed whorls and the appearance of the sutural shelf. This is illus- 

 trated in the specimens figured on plate I. 



