296 STUDIES IN EVOLUTION 



By observing the stages of development in the austral and 

 boreal terebratellids, it is seen that both start from a common 

 larval stage, and divergence into two lines begins in the first 

 adolescent stages, so that the series of metamorphoses in 

 each is quite distinct nearly to the end. This in itself might 

 not require that the austral and boreal species should be 

 referred to different genera and placed in different sub-fam- 

 ilies ; but when it is found that all the other southern genera 

 of the Terebratellidae represent arrested and degraded stages 

 in the development of a southern Terebratella or Magellania^ 

 and that the northern genera represent similar stages in the 

 development of a northern high type, such a separation neces- 

 sarily follows.* Moreover, these stages have a more profound 

 significance, as several of them in both regions represent 

 established genera now extinct. 



A feature which may be of service in distinguishing adult 

 recent shells is, that the Dallinina; have small cardinal proc- 

 esses, and the interior of the dorsal beak is usually grooved 

 to the apex, while in Magellaniinse there is a well-developed 

 projecting cardinal process often filling the cavity of the 

 beak. The lower genera can be readily determined by the 

 characters of the loop and by the median septum, which is 

 generally low in the Dallininfe and projecting above the loop 

 in the Magellaniinge. With these considerations in mind, the 

 metamorphoses and relations of the northern Terebratellidse 

 may be described. 



There are two finished types of northern genera, which are 



* Platidin seems to he an exception in the distribution of the northern genera, 

 as it has been recorded fron) Marion Island, in the southern Indian Ocean. The 

 northern forms referred to Mat/asella are without the characteristic higli septum 

 of M. Cumingi, and appear to be stages of development of a higher northern 

 form. 



In Fischer's "Manuel de Conchyliologie," p. 1246, (Ehlert, in discussing the 

 geographical distribution of brachiopods, says : " Parmi les Brachiopods il eu est, 

 dont la distribution est en rapport avee la temperature r^gionale ; c'est ainsi 

 qu'uu certain uombro d'esp^ces sont particulieres aux mers qui avoisincnt les 

 poles, chaque hemisphere ayant ses formes spe'ciales qui lui appartioniient en 

 propre, a I'exception de Terthralulina caput-serjtentis, var. se))tentr!rmalis, qui se 

 trouve h, la fois dans I'liemisphere austral et dans I'he'misphere boreal." 



