SCHUCHERT. 



CLASSIFICATION OF BRACHIAL STRUCTURES. 



109 



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

 exemplified in Thecidea, Terebratulina, and Rhyuchouella. 



PTYCHOLOPHUS STAGE. 



The simplest of the types of brachia just cited is 

 the schizolophus by the additional lol)atioii, or loopiii 

 lobes, makiug a structure which may be called 

 Megathyris aud Lacazella meditcrranea both have 4 

 Thecidea rddlaia has (>; T. rer- 

 mk'ularu and EudeseUa may- 

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

 phloios aud Oldhamiua, about 

 -0. Lobatiou in some (Theci- 

 dea) is produced by the forkiug 

 orbrauchiugof the median sep- 

 tum; in others (Pterophloios) 

 the septum remains simple 

 while the lateral borders of 

 the lophophore are lobed. 



ZUGOLOPHUS AND PLECTOLO- 

 PHUS STAGES. 



developed out of 



^■, of the primary 



the ptychiAophuH. 



lobes (fig. LV/, //); 



Taxolophus 



Trocholophus. 



Schizolophus. 



Zugolophus. 



Plectolophus. 



All the higher Terebratu- 

 lacea reach the final growth of 

 the loj^hophore through an in- 

 termediate stage which from its 

 form may be called the Zugolo- 

 Ijhus — fig. Sdj i. Eucala.his 

 and Platidia (?Tropidoleptus) 

 are apparently adult represen- 

 tatives of this stage, while 

 Kraussina and probably Bou- 

 chardia are slightly more ad- 

 vanced by the growth of a short 

 median, coiled arm, aud lead to 

 the next highest, or plectolo- 

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

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



A long loop pointed in front like Kenssehcria 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 Terebratellidip 

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

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

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

 resorption of the anterior portion the ascending branches are formed 



Fia. 3. — Stages of growth of the lophophore iu the 

 Terebratellidffi and Terehratnlidu'. n, b, c, d, e, five 

 stages in the development of the lophophore iu the 

 Terebratellidie. a-d, Terebratalia obsoleta, enl. (after 

 Beecher '). e, Magellania kerguelenensis, nat. size (after 

 Davidson'). /, jr, ft, t,j, development of lophophore in 

 the Tcrebratulidse. f-i, early stages in Terebratulina 

 $eptentrionalis, enl. (after Morse "). j, adult Terebratu- 

 lina canccllata (after Davidson"). 



