DEVELOPMENT OF THE BRACHIOPODA 283 



changes which have been described in detail elsewhere.* 

 These metamorphoses are of great assistance in understand- 

 ing the development and comparative morphology of this 

 feature in other groups of the Spiriferacea. The earliest 

 stage observed (figure 125, a) has the form of a simple tere- 

 bratuloid loop, which, from its resemblance to Centromella^ 

 was called the centronelliform stage. Since approximately 

 this form of brachidium is also characteristic of the young 

 of recent terebratuloids, it may be taken in Zygospira as 

 indicative of the trocholophus stage of brachial development. 

 With this as a starting-point for comparison, the further 

 correlation of the succeeding stages is very simple. 



The first resorption of the end of the loop in Zygospira 

 produced a schizolophus condition, and further resorption 

 carried the brachidium to a stage closely resembling Dielasma 

 (figure 125, K). The dielasmatiform stage has already been 

 explained as due to the requirements of space for the growth 

 of the coiled brachia. Next, the initial calcification of 

 the spiral arms resulted in the extension of the descending 

 branches beyond the jugum (figure 125, c), and, lastly, com- 

 plete calcification manifests the spirolophus structure and 

 produced the characteristic brachidium of the Spiriferacea. 



The AtrypidsB and the Athyridae seem to stand to each 

 other in the same relation as the Terebratellidfe and Tere- 

 bratulidse. In the first the descending branches are widely 

 separated and follow the edges of the valves; in the second 

 the descending branches are close together. This difference 

 in the Spiriferacea produces the converging cones of the 

 Atrypidae (figure 125, d) and the diverging cones of the 

 Athyridae, Spiriferidee (figure 125, g), etc. 



It seems doubtful whether the fleshy portions of the brachia 

 in the Meristellidae and Athyridse possessed additional char- 

 acters expressing the complexity and elaboration reached by 

 the jugal processes, even when the lamellae were duplicated, 

 as in Koninckina and Kayseria. 



From the foregoing descriptions and illustrations it appears 

 that the mode of growth of the cirrated lophophore, or 



