334 E. W. Berry — An American Spirulirostra. 



probable, would necessitate considering Spirulirostra as 

 a more evolved type than Spirula which had secondarily 

 acquired a rostrum and proostracum. This is an absurd- 

 ity for which the evidence is so rare, if indeed there is any 

 instance, that it has led to the formulation of what is 

 known as Dollo's law of the irreversibility of evolution. 

 I think it may be concluded that both Spirulirostra and 

 Spirula show vestiges of their more remote ammonite 

 ancestors in their protoconch and ventral siphon, but 

 that their more immediate ancestors were some unknown 

 belemnoid forms, and that Spirulirostra might well serve 

 as a prototype of the existing Spirula which subsequently 

 lost all traces of the guard and proostracum, features 

 whose loss may possibly be correlated with the modifica- 

 tion of body form from more graceful lines in the direc- 

 tion of the short, stout bodied and truncated hind end of 

 the existing Spirula. 



