294 STUDIES IN EVOLUTION 



dorsal valve.* The next stage was called the 2)'rcemaffadiform 

 stage (Plate XIV, figures Ca, Da\ but it may well be divided 

 into two stages, which correspond in structure to adult 

 Boucliardia and Megerlina. The houchardiform stage (figure 

 Ca) has a high quadrangular septum in the dorsal valve, and 

 on the posterior distal angle there is a small circle, or calca- 

 reous ring. The crura aie present, but the primary lamellae 

 have not yet appeared. In the next stage, the megerliniform 

 (figures Da, Da'), the ring has increased in size, and below, 

 on the septum, have appeared two projections or points, 

 which are the beginnings of the descending primaiy branches. 



The subsequent, or magadiform, stage (Plate XIV, figure 

 Ea) shows the completion of the descending branches to form 

 the primary loop, and also the enlargement of the secondary 

 loop or ring. During further growth the primary and 

 secondary loops approach each other on the septum, then 

 coalesce and make the magaselliform condition represented in 

 figure Fa. 



The ventrally projecting, free portion of the septum next 

 is absorbed, and the branches of the loop become attenuated, 

 but still the descending branches remain connected with the 

 septum, and thus the terehratelllform stage is completed 

 (Plate XIV, figure Go). 



Magellcmia venosa, after passing through all the stages 

 described, including the terehratelllform, loses the connecting 

 bands, and develops into the final magellnmform type of 

 structure (Plate XIV, figure Ha). Moreover, Magellania 

 lenticidaris, M. flavescens^ Terehratella cruenta, and T. ruhi- 

 cunda, as far as observed, correspond closely in their develop- 

 ment with the morphogeny of 31. venosa. 



A fact of importance noticed by Fischer and CEhlert^ is 

 that these species are confined to the southern hemisphere. 

 The other austral types of terebratuloids, exclusive of the 

 genera of Terebratulida?, as here restricted, are Magasella 

 (M. Cumingi), Kraussina, Megerlina, and Bouchardia. In 



* An earlier gwi/niform stage has been observed by the writer in a young 

 example of Mugellania Jlavesceus. 



