288 STUDIES IN EVOLUTION 



tare. Generic characters in this family are usually based 

 upon the form and disposition of the brachia and their sujj- 

 ports. The highest genera in one sub-family, which is austral 

 in distribution, pass through stiiges correlated with the adult 

 structure in the genera Gwi/nia, Cistella, Bouchardia, Meger- 

 U7ia, Magas, 3Iagasella, and Terebratella, and reach their 

 final development in Magellania and Neothyris. The higher 

 genera in another sub-family, boreal in distribution, pass 

 through metamorphoses correlated with the adult structures 

 of Grwynia, Cistella, Platidia, Isnienia, 31ilhlfeldtia, Terehra- 

 talia, and Dallina. The first two stages in both sub-families 

 are related in the same manner to Crivynia and Cistella. The 

 subsequent stages are different except the last two, so that 

 the Magellania structure is similar in all respects to the Dal- 

 lina structure, and Terehratella is like Terehratalia. There- 

 fore Magellania and Terehratella are respectively the exact 

 morphological equivalents to, or are in exact parallelism 

 with, Dallina and Terehratalia. 



The stages of growth of the genera belonging to the two 

 sub-families Dallininre and Magellaniinae are further corre- 

 lated in the tables on page 303. 



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

 through no brachial metamorphoses, and has the same struc- 

 ture 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 acceler- 

 ated to the immature period, and in Ismenia (representing an 

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

 form and cistelliform stages are larval, and the j^latidifortn 

 represents 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 imma- 

 ture stage of the next higher, until the highest member in its 

 ontogeny represents serially, in its stages of growth, all the 

 adult structures, witli the larval and innnature stages of the 



