Loop-bearing Brachiopoda. 383 



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 

 condition was correlated with the genus Flatidia, by Deslong- 

 champs,^ and was called the platidiforin stage. It has also been 

 called the centronelUfonn stage by Fischer and OEhlert,^ 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 1, figure Dl, 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 

 El, called the Umeiiiform stage. Lacunae are then produced by 

 resorption in the broad plates forming the ascending branches, and 

 the structure of the supports at this time, figure Fl, 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 miXhlfeldtiform stage. A further broaden, 

 ing of the loop and completion of the structures already outlined, 

 with the recession of the secondary connecting bands from the sep- 

 tum, results in Laqueus, figures G4, Go, which has connecting bands 

 from the ascending to the descending lamellae and from the latter 

 to the septum.* 



The absorption of the connecting bands from the ascending 

 branches completes the Terebratalia stage, Plate I, figures Gl, G2, 

 in Macandrevia and DalUna, and is the adult condition in Tere- 

 bratalia transversa, T. obsoleta, T. frontalis, and T. coreanica, 

 figure G3.f 



Finally the resorption of the connecting bands from the descend- 

 ing branches produces the Dallina structure, and the further resorp- 

 tion of the septum terminates the series in Macandrevia, Plate I, 

 figure Hi. 



Comparisons and homologies. 



Thus the genera of the Terebratellidae begin their larval develop- 

 ment in a form like Gwynia, having no calcified brachial supports, 

 and with a simple circle of centripetally directed tentacles. Then 



* The name Megerlina Jeffreysi was given to a stage of Laqueus caUfornica from its 

 having a structure like Megerlina {= Miihlfeldtia) truncata, thus indicating clearly the 

 close relationship of these genera. 



f T. spitzbergensis and T. Marioi, from the unfinished appearance of their brachial 

 supports, possibly will be found to belong to a higher member of the series : e. g. 

 Dallina. 



