386 (7. E. Beecher — Revision of the Families of 



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 tere- 

 hratelliforru structure, Plate I, figures Ga-GcZ, comparable directly 

 with figures G1-G6 of the Dallininae or boreal genera. Also, as 

 in the Dallininge, the disappearance of the connecting bands com- 

 pletes the magellaniforni stage, and terminates the series, figures 

 Ra-He. 



The stages of growth of the genera belonging to the three sub- 

 families of the Terebratellidse, the Megathy rinse, Dallininse, and 

 Magellaniinse, are further correlated in the accompanying tables- 

 It must be understood, of course, that the larval and immature 

 stages have not been observed in all the genera, but froni the 

 known ontogeny of several of the lo^^*er and higher forms, and 

 from evident homologies of structure, such stages may be inferred. 



Morphogeny from Gwynia to Megathyris. 



Periods. 



Stages. 



Stages. 



Stages. 



Larval 



Adolescent 



Mature 



gwyniform ? 



gwyniform 



Gwynia 



gwyniform 

 cistelliform 

 Cisfella 



gwyniform 

 cistelliform 

 Megathyris 



The simplest genus, Gwynia^ as far as known, passes through no 

 metamorphoses, and has the same structure throughout the adoles- 

 cent period, up to and including the mature condition. Li the onto- 

 geny of (Jistella, the gioyniform stage through acceleration has 

 become a larval condition. In Platidia^ the cistelliform structure 

 is accelerated to the immature period, and in Ismenia (representing 

 an ismeniforra type of structure in the higher genera), the gwyni- 

 form and cistelliform stages are larval, and the platidiform repre- 

 sents an adolescent condition. Similar comparisons may be made 

 in the other genera. Progressively through each series, the adult 

 structure of any genus forms the last immature stage of the next 

 higher, until the highest member in its ontogeny represents serially, 

 in its stages of growth, all the adult structures, with the larval and 

 immature stages of the simpler genera. 



