298 STUDIES IN EVOLUTION 



of the septum is the same as that described in G-wynia by 

 King. The brachia form a slender fleshy ellipse or circle, 

 resting in front on the floor of the interior of the dorsal 

 valve, with the tentacles or cirri centripetal or directed 

 inward, as in an early stage of Cistella. After this gwyni- 

 form stage the growth of the septum inflects the circlet of 

 tentacles, producing a condition identical with that in adult 

 Cistella (Plate XIV, figure B). It is therefore called the 

 cistelliform stage. 



The succeeding transformations in Dallina septigera and 

 Macandrevia cranium have been fully described by Friele. 11 

 These species, with Terebratalia obsoleta Dall, sp., make three 

 typical northern forms whose development has been observed. 

 They agree in every essential detail, and may be described 

 in general terms. The first stage described by Friele (Plate 

 XIV, figures Ci, Di), showed the growth of the descending 

 lamellae, their union with the septum, and the appearance of 

 a small ring on the top of the septum, which is the beginning 

 of the ascending branches, or secondary loop. This condi- 

 tion was correlated with the genus Platidia, by Deslong- 

 champs, 7 and was called the platidiform stage. It has also 

 been called the centronelliform stage by Fischer and QEhlert, 8 

 but, as Centronella is not known to have a septum supporting 

 the loop, the name is not adopted here. 



The lower anterior part of the secondary loop begins to 

 divide very early (Plate XIV, figure Di), and, at the same 

 time, the ends of the descending branches broaden and 

 approach the top of the septum, being thus in juxtaposition 

 to the ascending branches (as in figure Ei), called the ismeni- 

 form stage. Lacunse are then produced by resorption in the 

 broad plates forming the ascending branches, and the struc- 

 ture of the supports at this time (figure Fi), resembles that 

 in adult Muhlfeldtia sanguinea and M. truncata (figure F3), 

 in which the secondary loop is still attached to the septum. 

 This stage is here termed the muhlfeldtiform stage. A further 

 broadening of the loop and completion of the structures 

 already outlined, with the recession of the secondary connect- 



