PHYLOGENIC STAGE OF THE GASTROPODA 4^3 



species than he describes. The "various degrees of uncoih'ng" 

 are rather within a species in each case. That they are gerontic 

 is, secondly, not assured. 



Returning to the early Cambrian shells, the feature, in long 

 cones, of the apical region coiling more than the maturer por- 

 tion of the shell does is not necessarily to be interpreted as 

 "uncoiling " in a genetic sense, but is as well or better to be con- 

 sidered as due to the curvature having set in at the apical region, 

 and thence affecting the entire shell. Similarly, the reverse cur- 

 vature is seen to affect the apex of Tryblidiuni retrorsiim Whitf. 

 (Fig. 5.), and the entire shell of Hypseloconus reciirva Whitf. 

 (Fig. 6), the former being then the transition from the normally 

 curved long cones to the recurved short cones of the latter. 



In all cases the change from straight to curved and coiled 

 shell is accompanied by a change from directly transverse to 

 obliquely cut back aperture in a very natural manner, as if the 

 curvature of the shell preceded the elongation of the anterior, 

 i. e., the convexed side of the cone. 



The next noteworthy feature is that, while the conical shells, 

 which are very variable species, appear in each fauna, the coiled 

 spiral shells are dextral in the eastern Olenellus fauna and sinis- 

 tral in the Baraboo fauna. In this case the sinistral shells are 

 evidently not sinistral derivatives of dextral ancestors by the 

 depressing of the apex, as once suggested by Billi-ngs, below the 

 median plane, since such transitions are unknown. A coiled 

 intermediate type between the Raphistoma and the Scaevogyra 

 does not appear, and coiling in bilateral symmetry was seemingly 

 not yet developed or not yet immigrated. The link from the 

 sinistral to the dextral shells may be interpreted as the known 

 long conical type, and the two kinds of spiral shells as arising 

 by parallelism. The sinistral might also be simply a mysteriously 

 reversed form of the dextral, though they do not appear to be 

 the sinistral individuals, or the near reverse of any dextral form, 

 but relate rather to the retrorse long conical, Hypseloconus. 



THE CALCIFEROUS FAUNA. 



The next succeeding fauna is in the Calciferous. This group 

 may be considered as descended in main from the earlier types 



