PHYLOGENIC STAGE OF THE GASTROPODA 47 ^ 



The conical shells of other Gastropoda, t. e., those among 

 Pulmonata and Ophistobranchia, are presumably vestigial from 

 spiral ones, and, moreover, are historically distinct from the 

 Cambrian fauna. The Docoglossa are, therefore, seemingly 

 the only ones without known coiled or spiral-shelled ancestor, 

 and any such ancestor must have existed, if at all, before Cam- 

 brian time. 



The question is, then, whether the Docoglossa had evolved 

 their shells prior to others through all the stages from the 

 primitive conical through the spiral to the derived conical form, 

 or whether they have had no other than the conical shell as yet. 

 It is not necessary to assume that the Docoglossa had a spiral- 

 shelled ancestor. The aspect of the Cambrian fauna, indeed, 

 suggests that such a theory is not the clue to their evolution. And 

 if it could be proved that the Docoglossa had not a coiled or 

 spiral-shelled ancestor, the dilemma could be removed from the 

 interpretation of Cambrian Gastropoda. It seems at least 

 desirable to avoid the set conclusion that the Docoglossa had 

 spiral-shelled ancestors, and this is practicable only by using a 

 conservative view as to the theory of the visceral twisting and 

 by changing the above-cited theory of the spiral-shelled ancestor. 



In support of a new theory, it may further be noted here that 

 the Fissurella type of Rhipidoglossa does not appear early 

 enough in geologic time nor in such association as to indicate 

 the probability of truth in the view of the apical slit of its shell' 

 having any homology with the similar structure in Dentalium of 

 the class Scaphopoda. The known early Paleozoic gastropods 

 have not even the marginal slit. 



Judging from comparison of the living gastropods, it is 

 obviously not to be assumed that the phylogenic groups or lines 

 in which the spiral shell has longest persisted have the visceral 

 asymmetry most strongly developed ; but rather the contrary 

 would seem to be the case. It may even be taken that an asym- 

 metrical long cone, while developing phylogenically, was the 

 suflficient cause for the asymmetrical twist of the viscera as seen 

 now in Gastropoda, and that the short conical shell, on the one 



' Op. cit., p. 83. 



