FAMILIES OF LOOP-BEARING BRACHIOPODA 301 



form stage of Macandrevia, Dallina, Laqueus, and the adult 

 structure of Muhlfeldtia sanguined. After this, the union of 

 the primary and secondary loops and their removal from the 

 septum to which they remain attached only by connecting 

 processes form a structure like that in Laqueus (figures G*, 

 Gs), and the resorption of the connecting bands from the 

 ascending branches of the loop completes the terebrataliform 

 stage of Macandrevia and Dallina, as shown in Plate XIV, 

 figures Gi, G2. Terebratalia is the present fixed genus of 

 this type of structure (figure Gs), and Trigonosemus (figure 

 G5), is a Cretaceous representative. Finally, by the resorp- 

 tion of the bands of the terebrataliform stage, the structure 

 of the highest genera, Macandrevia and Dallina, is reached 

 (figures Hi-H5). 



The first stage after the cistelliform in the Magellan iinse, 

 the austral branch of the Terebratellidse, is represented for 

 Terebratella dorsata, in Plate XIY, figure Ca. Kraussina 

 (figures Cb, Cc) has a simple fork or V-shaped process on the 

 septum, which apparently represents an incomplete secondary 

 loop. The relations of Bouchardia (figure Ccf) to this 

 bouchardiform stage of Terebratella are more evident. After 

 this stage the beginnings of the primary loop, or descending 

 branches, appear as two projections on each side of the 

 septum (figure Da'). Megerlina (figure D5) shows this ad- 

 vance over Kraussina. 



The completion of the descending branches in the next, or 

 magadiform, stage is represented for Terebratella dorsata, in 

 Plate XIV, figure Ea ; T. cruenta, figure EC ; T. rubicunda, 

 figure E<# ; Neothyris lenticularis, figure E> / Magasella Cum- 

 ingi, figure E/. The Cretaceous equivalent, Magas, is shown 

 in figure Ee. In all these forms the septum projects above 

 the descending lamellae nearly to the ventral valve. 



T. Buckmani Moore, figure Fe). A strict interpretation of that genus based upon 

 T. dorsata, the type, excludes these species, which agree with the definition of 

 Ismcnia in that the ascending and descending branches are attached directly to 

 the septum. They may be, however, stages of growth of higher forms. 



