lyS PRIMARY FACTORS OF ORGANIC EVOLUTION. 



The two chief subfamilies of the Terebratellidae 

 undergo complicated series of metamorphoses in their 

 brachial structure. Generic characters are based upon 

 the form and disposition of the brachia and their sup- 

 ports. The highest genera in one subfamily, which is 

 austral in distribution, pass through stages correlated 

 with the adult structure in the genera Gwynia, Cistella, 

 Bouchardia, Megerlina, Magas, Magasell, andTerebra- 

 tella, and reach their final development in Magellania 

 and Neothyris. The higher genera in another subfam- 

 ily, boreal in distribution, pass through metamorphoses 

 correlated with the adult structures of Gwynia, Cistella, 

 Platidia, Ismenia, Miihlfeldtia, Terebratalia, and Dal- 

 lina. The first two stages in both subfamilies are re- 

 lated in the same manner to Gwynia and Cistella. The 

 subsequent stages are different except the last two, so 

 that the Magellania structure is similar in all respects 

 to the Dallina structure, and Terebratella is like Tere- 

 bratalia. Therefore Magellania and Terebratella are 

 respectively the exact morphological equivalent to, or 

 are in exact parallelism with Dallina and Terebratalia. 



The stages of growth of the genera belonging to 

 the two subfamilies Dallininae and Magellaniinae are 

 further correlated in the accompanying tables. 



The simplest genus Gwynia, as far as known, passes 

 through no brachial metamorphoses, and has the same 

 structure throughout the adolescent period, up to and 

 including the mature condition. In the ontogeny of 

 Cistella the gwyniform stage, through acceleration, has 

 become a larval condition. In Platidia, the cistelliform 

 structure is accelerated to the immature period, and 

 in Ismenia (representing an ismeniform type of struc- 

 ture in the higher genera), the gwyniform and cistelli- 

 form stages are larval, and the platidiform represents 



