scHucHERT] CLASSIFICATION OF BRACHIAL STRUCTURES. 



109 



three characteristic types of brachia of all the higher brachiopods. as 

 exemplified in Thecidea, Terebratuliaa, and Rhyuchouella. 



PTYCHOLOPHUS STAGE. 



The simplest of the types of brachia just cited is developed out of 

 the schizolophus by the additional lobatiou, or looping, of the primary 

 lobes, making a structure which may be called the pfycholophus. 

 Megathyris and Lacazella mediterranea both have 4 lobes (fig. 2d, h); 

 Thecidea radiata has 6 ; T. ver- 



f 





Taxoloplius. 



Trocholo])hus. 



Schizolophus. 



Zugolophus. 



micularis and EudeseUa may- 

 ale, 8; E. digitata, 10; Ptero- 

 phloios and Oldhamina, about 

 20. Lobation in some (Theci- 

 dea) is produced by the forking 

 orbranching of the median sep- 

 tum; in others (Pterophloios) 

 the septum remains simple 

 while the lateral borders of 

 the lophophore are lobed. 



ZUaOLOPHUS AND PLECTOLO- 

 PHUS STAGES. 



All the higher Terebratu- 

 lacea reach the final growth of 

 the lophophore through an in- 

 termediate stage which from its 

 form may be called the Zugolo- 

 plms — fig. M, i. EucalaJiis 

 and Platidia (?Tropidoleptus) 

 are apparently adult represen- 

 tatives of this sfage, while 

 Kraussina and probably Bou- 

 chardia are slightly more ad- 

 vanced by the growth of a short 

 median, coiled arm, and lead to 

 the next highest, or jilectolo- 

 phus, stage, in which there is a well-developed spiral arm with a 

 fringe of cirri on each edge — fig. 3e, /. 



A long loop pointed in front like Kensselseria and Ceutronella could 

 not have supported a median arm, as the pallial cavity is thus fully 

 occupied, and the development of the brachidium in the Terebratellida) 

 shows that the central space between the branches of the loop is to 

 accommodate such an organ. The same is doubtless true of Dielasma, 

 which first has a Centronella-like loop, and through the subsequent 

 resorption of the anterior portion the ascending branches are formed 



Plectolophus. 



Fid. 3.— Stages of growth of the lophophore in the 

 Terebratellidas and Terebratnlid:i'. a, &, c, d, e, five 

 stages in the development of the lophophore in the 

 TerebratellidcB. a-d, TerebrataUa obsoleta, eul. (after 

 Beecher^). e, Magellania kergnelenensis, nat. size (after 

 Davidson'). /, i;, /;, i,j, development of lophophore in 

 the Torebratulidae. /-i, early stages in TerehratuUna 

 septentrionalis, enl. (after Morse ") . j, adult Terehratu- 

 Una cancellata (after Davidson '). 



