278 STUDIES IN EVOLUTION 



Troeholophus Stage. 



By the continual addition of new cirri and the pushing 

 back of the old ones, the fringed margin of the lophophore 

 passes from a crescentic to a circular form, thus making a 

 complete ring about the mouth. This may be termed the 

 troeholophus stage. It appears in the late larval and early 

 adolescent stages of Thecidea (figure 121, 5), Cistella (figure 

 \21,f'),Mag6llania&nA Terabratalia (figure 122, 6), Terehratu- 

 lina (figure 122,^), Gilottidia (figure 124, 5), and Disoinisca, 

 and, like the former stages, is undoubtedly common to all 

 brachiopods, except, perhaps, Iphidea. 



Gwynia is an adult living representative of this stage, and 

 never develops any higher type of brachial structure. Dys- 

 colia also belongs here, since it has a discoid lophophore 

 surrounded by a marginal fringe of tentacles (Fischer and 

 CEhlert^). It is possibly a little more advanced than G-wynia, 

 as it has a slight median anterior notch, suggesting the begin- 

 ning of- the bilobed structure of the next higher type. 



The absence of septum, hinge-plate, and dental plates are 

 other primitive characters belonging to Dyscolia. 



Schizolophus Stage. 



After the completion of the troeholophus stage in all brachi- 

 opods, except such simple forms as Gwynia and Dyscolia, no 

 further increase in the cirrated edge of the lophophore can 

 occur without some deformation of the circle. This is first 

 accomplished by an introversion of the anterior median edge, 

 thus dividing the lophophore into two lobes, and suggesting 

 the name schizolophus for this type. (See figures 121, e, g, 

 122, c, h, 124, c.) 



Several brachiopods retain the schizolophian brachia as 

 an adult character. Of these, Cistella is perhaps the best 

 example, as it agrees exactly with an early stage of arm 

 structure among the Terebratellidse, which has been called 

 the cistelliform stage (figure 122, c). Terebratulina (figure 



