VOL. VIII, PP. 71-82, PLS. X, XI JULY 13, 1893 



PROCEEDINGS 



BIOLOGICAL SOCIETY OF WASHINGTON. 



DEVELOPMENT OF THE BRACHIAL SUPPOPxTS IN 

 DIELASMA AND ZYGOSPIRA. 



BY CHARLES E. BEECHER AND CHARLES SCHUCHERT. 



It lias been shown by several authors* that the brachial sup- 

 ports in the Terebratellida) pass through a series of distinct 

 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 ontogerietic series of 

 considerable completeness, and it is often difficult or impossible 

 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 structure so delicate 

 as the calcareous lamellae supporting the brachia, especially in 

 young specimens from one to five millimeters 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 Ancylo- 

 brachia. For this purpose, a species of Dielasma (D. turgida) 

 obtained from Mr. Moritz Fischer was used. The specimens are 



* Davidson, Friele, Deslongchamps, Fischer and (Ehlert, and Beecher. 



lo-Bioi,. Soc. WASH., VOL. VIII, 1893. (71) 



