THE PHYLOGENIC STAGE OF THE CAMBRIAN 

 GASTROPODA. 



(with plates I AND II,) 



Whether it is due to imperfection of the geologic record of 

 the Cambrian age, or whether it is because the class Gastropoda 

 was at that time small and at an initial stage of its differentiation, 

 the species of that age, as known to us, are few, simple-structured, 

 and little diverse, as compared with those of later geologic ages. 

 From the looks of the fossils it might, indeed, be granted that 

 the fauna is both imperfectly represented and primitive. But, as 

 well known, the very simplest and most primitive-looking ones 

 are referable to families the living members of which are believed 

 by zoologists to be quite remote from the initial or primitive 

 type of the class. If their theory is correct, then the Cambrian 

 Gastropoda must be considered as not near the initial stage of 

 its differentiation, but far advanced, and the Cambrian geologic 

 record as very incomplete. On the other hand, it has not been 

 found explicable why the Cambrian record is so peculiarly and 

 greatly incomplete, and, in short, the primitive looks of the 

 Cambrian Gastropoda are not geologically explained, and the 

 question returns to the interpretation which zoologists have made 

 of the Recent animal and its shell, whether this is not somewhat 

 at fault for the dilemma. The following paragraphs are intended 

 to offer a concordant interpretation of the geologic and biologic 

 evidence on the Cambrian Gastropoda. 



According to the generally accepted view, the gastropod 

 shell, which is the modification of a hollow cone in structure, 

 arose from a symmetrical conical tegulum, and thence the 

 conical, curved conical, spirally coiled, and plane coiled types 

 of shell were successively evolved. The development of the 

 spiral shell has been accompanied by, or has been the cause' of, 

 a twisting of the mantle complex of the animal, and since the 



' See Arnold Lang, Lehrbucli der vergleichemien Anatomie (Zweite Aufiage, 

 igoo), pp. 81-7. 



469 



