7. DEVELOPMENT OF THE BRACHIAL SUP- 

 PORTS IN DIELASMA AND ZYGOSPIRA* 



(PLATE XXVI) 



It has been shown by several authors f that the brachial 

 supports in the Terebratellidse pass through a series of dis- 

 tinct metamorphoses during the life of the animal. In the 

 higher genera these stages may be correlated with the adult 

 structures of lower forms, thus furnishing satisfactory data 

 for a systematic arrangement of the genera and for their 

 phylogenetic relations. 



This kind of research naturally requires ontogenetic series 

 of considerable completeness, and it is often difficult or im- 

 possible to obtain such material representing fossil forms. 

 Moreover, the fossils must be exceptionally well preserved to 

 afford a means of working out the development of a struc- 

 ture so delicate as the calcareous lamellae supporting the 

 brachia, especially in young specimens from one to five 

 millimetres in length. 



It first seemed desirable to determine the development in 

 some genus of the Terebratulidse from the Paleozoic, in order 

 to ascertain whether the brachial supports, as in Neozoic and 

 recent forms, passed through a series of transformations, and 

 to determine the most primitive form of the loop in the 

 Ancylobrachia. For this purpose a species of Dielasma 

 (D. turgidum) obtained from Mr. Moritz Fischer was used. 

 The specimens are from the St. Louis Group of the Lower 

 Carboniferous in Kentucky. The shells are partially silici- 



* Beecher and Schuchert. Proc. BioL Soc. Washington, VIII, 71-78, pi. x, 

 1893. 



t Davidson, Friele, Deslongchamps, Fischer and CEhlert, and Beecher. 



