Loop-hearing Brachiopoda. 385 



feldtia^ as well as the final condition of Ismeyila^ and figures 

 F1-F4 represent the muhlfeldtiform stage of Macandrevia, Dal- 

 lina, Laquerts, and the adult structure of Muhlfeldtia sangiiinea. 

 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, forms a structure like that in Laqueus, figures 

 G4, G5, and the resorption of the connecting bands from the ascend- 

 ing branches of the loop completes the terehratcdiform stage of 

 Macandrevia and Da.llina, as shown in Plate I, figures Gl, G2. 

 Terebratalia is the present fixed genus of this type of structure, 

 figure G3, and Trigonosemus, figure G5, is a Cretaceous representa- 

 tive. Finally by the resorption of the bands of the terehratalifonn 

 stage, the structure of the highest genera, Macandrevia and Dallina^ 

 is reached, figures H1-H5. 



The first stage after the cistelliform in the Magellaniinse, the 

 austral branch of the Terebratellidte, is represented for Terehratella 

 dorsata, in Plate I, figure Ca. Kraussina^ figures C&, Cc, has a 

 simple fork or V-shaped process on the septum, which apparently 

 represents an incomplete secondary loop. The relations of Bou- 

 chardia, figure Cd, to this houchardiform stage of Terehratella, are 

 more evident. After this stage, the beginnings of the primary loop, 

 or descending branches, appear, as tw^o projections on each side of 

 the septum, figure J^a' . Megerllna, figure D^, shows this advance 

 over Kraussiyia. 



The completion of the descending branches in the next, or onag- 

 adiform, stage is represented for Terehratella dorsata, in Plate I, 

 figure Ea ; T. cruenta, figure Ec ; T. rnhicunda,, figure Ec// N'eo- 

 thyris lenticidaris, figure EJy Magasalla Cumingl, figure E/". The 

 Cretaceous equivalent, Mag as, is shown in figure Ee. In all these 

 forms, the septum projects above the descending lamellae nearly to 

 the ventral valve. 



After the magadiform stage, the descending and ascending 

 branches approach and unite, and at the same time there is a nar- 

 rowing of the latter, Plate I, figures Ya-Fe. 3Iagasella Cumingi 



* Figures F5 and F6, Plate I, are from Davidson.^ They are Jurassic species and 

 were referred to Terehratella {T. fttrcata Sow., figure F5, and T. Buckmani Moore 

 figure F6). A strict interpretation of that genus based upon T. dorsata, the type, 

 excludes these species, which agree with the definition of Ismenia in that the ascend- 

 ing and descending branches are attached directly to the septum. They may be, how- 

 ever, stages of growth of higher forms. 



