302 



STUDIES IN EVOLUTION 



After the magadiform stage the descending and ascend- 

 ing branches approach and unite, and at the same time 

 there is a narrowing of the latter (Plate XIV, figures Fa-Fe). 

 Magasella Cumingi Davidson seems to be the only permanent 

 adult representative of this structure which has yet been 

 found. 



The further narrowing of the lamellae, broadening of the 

 loop, and absorption of the free portion of the septum, result 

 in the terebratelliform structure (Plate XIV, figures Ga-Gd), 

 comparable directly with figures Gi-G6 of the Dallininae or 

 boreal genera. Also, as in the Dallininse, the disappearance 

 of the connecting bands completes the magellaniform stage, 

 and terminates the series (figures Ha-He). 



The stages of growth of the genera belonging to the three 

 sub-families of the Terebratellidse, the Megathyrinse, Dal- 

 lininse, and Magellaniinse, are further correlated in the accom- 

 panying tables. It must be understood, of course, that the 

 larval and immature stages have not been observed in all the 

 genera, but from the known ontogeny of several of the lower 

 and higher forms, and from evident homologies of structure, 

 such stages may be inferred. 



Morphogeny from Gwynia to Megathyms. 



The simplest genus, Gwynia, as far as known, passes 

 through no metamorphoses, and has the same structure 

 throughout the adolescent period, up to and including the 

 mature condition. In the ontogeny of Cistella the gwyni- 

 form stage through acceleration has become a larval condi- 

 tion. In Platidia the cistelliform, structure is accelerated 

 to the immature period, and in Ismenia (representing an 

 ismeniform type of structure in the higher genera), the gwyni- 



