DEVELOPMENT OF THE BRACHIOPODA 281 



branching of the median septum; in others (^Pterophloios) 

 the septum remains simple, while the lateral borders of 

 the lophophore are lobed. 



Zugoloplius and PlectolopJius Stages. 



All the higher Terebratulacea reach the final growth of the 

 lophophore through an intermediate stage which from its 

 form may be called the zugolophus (figure 122, cZ, i). Eucala- 

 this and Platidia (? Tropidoleptus) are apparently adult rep- 

 resentatives of this stage, while Kraussi7ia and probably 

 Bouchardia are slightly more advanced by the growth of a 

 short median, coiled arm, and lead to the next higher, or 

 plectolophus, stage, in which there is a well-developed spiral 

 arm with a fringe of cirri on each edge (figure 122, e, j"). 



A long loop pointed in front, like Mensselceria and Centro- 

 nella, could not have supported a median arm, as the pallial 

 cavity is thus fully occupied, and the development of the 

 brachidium in the TerebratellidtB 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-\\kQ loop, and through the subsequent 

 resorption of the anterior portion the ascending branches 

 are formed and space allowed for the median arm (figure 

 123, a-d). In a spire-bearing genus like Zygospira this is 

 more obvious, for here the transverse process or jugum is 

 clearly the result of the growth and resorption of the cen- 

 tronelliform loop to admit the spiralia. 



123 



Figure 123. — Metamorphoses of the brachidium in Dielasma turgidum. 

 Enlarged. (After Beecher and Schuchert.) 



The calcareous loop in Terehratulina and Liothyrina is 

 only a posterior basal support, and does not repeat the out- 



