472 FREDERICK IV. SARDESON 



hand, and spiral shell, on the other, mark restored equilibrium. 

 A spiral or complete coil may never have existed in the case of 

 the Docaglossa. 



THE TIME SEQUENCE AND FORM SERIES. 



If we go back in theory to an ancestral mollusc with solid- 

 layered tegulum, the stages from that to the marginal growth, 

 and consequent hollow enlarged conical shell, seen in the sim- 

 plest Cambrian mollusc fossils, may be as many as from the 

 latter to the most modified Recent form of it ; and the first 

 conical Cambrian shell must well be called gastropod, and con- 

 sidered as far removed from the mollusc common ancestor. 

 How much farther advanced it was may be discussed. Consid- 

 ering that the known Cambrian gastropod shells, in a morpho- 

 logic sense, may be, some of them, more ancestral or primitive 

 than others, some species or genera should be found among 

 them, as in the later faunas, which were surviving ancient types. 

 Just as the genus Pleurotomaria ranges from Upper Cambrian to 

 Recent time, so also Cambrian genera may have ranged from 

 pre-Cambrian, and the first known fauna may represent an evo- 

 lutionary series. As said, the long, curved cone is the one which I 

 prefer to consider as the most ancestral type of Cambrian gastro- 

 pod shell, and the series, on the one hand, to the simple spirals 

 and, on the other, to the short, low conical form, is one-half as 

 long an evolutionary change as from the low conical to the long 

 conical, and to the spiral coil would be. To add, then, from 

 the spiral coil, to the coiled apex, to the secondarily low conical 

 shell, again doubles the length of evolutionary change. In short, 

 the evolutionary series, as represented by the Cambrian fossils, 

 is about one-fourth as long, considering the long curved cone as 

 ancestor of the short cones, as it is if the spiral is taken to pre- 

 cede the derived short cones among Docoglossa. 



The time sequence and form series of the Cambrian fossils 

 should indicate the course of development. But that the known 

 Cambrian fauna should not be taken as geologically complete is 

 indicated by the sudden appearance of the first genera. And 

 since the genera known in the Lower pass into the Upper Cam- 



