Vol. VIII, pp. 71-82, pls. X, XI July 13, 1893 



PROCEEDINGS 



1 1 F i' hi: 



BIOLOGICAL SOCIETY OF WASHINGTON. 



DEVET.OP^fENT OF THE BRACHIAL SUPPORTS IN 

 DIELASMA AND ZYGOSPIRA. 



BY CH.\RL?]S E. BEECPIER AND CHARLES SCIKirTIERT. 



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

 ports ill tlie 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 ontogenetic series of 

 consideraljle completeness, and it is often difficult or impossible 

 to obtain such material representing fossil forms. jVForeover, 

 the fossils must be exceptionally well preserved to afford a 

 means of working out the development of a structure so delicate 

 as the calcareous lamclLc sui)porting the btachia, especially in 

 young specimens from one to five millimeters in length. 



It first seemed desirable to determine the development in 

 some genus of the Terebratulida^ from the Paleozoic, in order 

 to ascertain whether the brachial supports as in Neozoic and 

 recent forms passed through a series of ti;ansformations, and to 

 determine the most primitive form of the loop in the Ancylo- 

 l)rachia. For this purpose, a species of Dielasma (D. lurgUla) 

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



'" Davidson, Friele, Deslongchamps, Fischer and QShlert, and Beecher. 



lii-Bioi,. Soc. Wash., Vol. VII I, ISit.'i. (71) 



