306 STUDIES IN EVOLUTION 



As for the long-looped Mesozoic forms, it is evident that 

 many of them underwent metamorphoses in their develop- 

 ment while attached to a dorsal septum. The presence with 

 them of such genera as Kinyena, Ismenia, and Milhlfeldtia^ 

 points to their intimate relations with the boreal stock of the 

 Terebratellidse, and therefore indicates that the Mesozoic 

 species with long recurved loops will in all probability be 

 found to agree with Macandrevia, Eudesia, Dallina, Trigo- 

 7iosemus, Li/ra, Terebratalia, Laqueus, Kingena, Ismenia, and 

 Millilfeldtia. Magas is exceptional, as it belongs to the 

 austral group of genera, and suggests the likelihood that 

 some of the higher species and genera associated with it 

 geologically may prove to belong to the same branch. 



The septum in the Terebratellidse appears before the 

 branches of the loop, and is an important character in the 

 series of metamorphoses until the last stage. At first it is 

 merely a support to the introverted growing cirrated portion 

 of the lophophore. This elongation of the cirrated margin 

 next results in an arch on the septum in the platidiform and 

 ismeniforiii stages. Further elongation produces the median 

 coiled arm, and the structures impeding its growth become 

 resorbed, so that the transverse space between the lamellae 

 widens, the connecting bands and high septum recede, and 

 finally even these disappear in Macandrevia and Magellania. 



The following table represents the genera arranged accord- 

 ing to the views expressed in this paper: — 



Family Terebratulid^ Gray. 



Loop free, developed by the growth and modification of 

 two primary lamellse. Cirri directed outward in larval 

 stages. 



Centronellin^ Waagen. 



Loop composed of two descending lamellse uniting in the 

 median line, forming a broad arched plate. 



Centronella Billings. Juvavella Bittner. 



Bensselceria Hall. Nucleatula Zugmayer. 



Netvherria Hall. 



