THE BRACHIOPODA 13 



genesis, for it has been shown by Beech'er and Thiele 

 that the loops of the modern terebratuloids Magellania and 

 Macandrevia, though much alike in the adult stage, pass 

 through two quite different series of metamorphoses, and 

 in each case the successive stages correspond to the 

 adult loop in a number of other genera, some still living, 

 others extinct (Fig. 9). 



It will be convenient here, before returning to the 



6. 



FIG. 4. HEMITHYRIS PSITTACEA (CHEMNITZ). 

 Recent, x-|. 



a, Ventral ; b, dorsal valve; Add., adductor muscle-impressions ; C, crus ; 

 Del., delthyrium, the deltidial plates removed; Div., divaricator 

 impressions; D.P., dental plates; S, socket; T, hinge -tooth. 

 (Original.) 



fossil forms, to consider some of the other brachiopods 

 found living in the sea. 



Hemithyris psittacea (Fig. 4) is a form living in the cold 

 northern waters : its shells are often cast up by storms on 

 the coast of Labrador. It agrees with the terebratuloids 

 in having dissimilar symmetrical valves, the larger being 

 perforated for a pedicle, and in having the delthyrium 

 partly closed in by a pair of deltidial plates. But these 

 plates, instead of bounding the foramen on its anterior 

 side only, bound it laterally and even tend to meet on 

 the posterior side just under the umbo (as in Fig. 9). On 



