BRACHIAL SUPPORTS IN DIELASMA AND ZYGOSPIRA 411 



fied, generally filled with transparent calcite, and afford very 

 satisfactory preparations of the arm supports. It was found 

 that the loop of Dielasma underwent transformations during 

 growth, and that the earliest stage observed is like Centro- 

 nella. This establishes the centronelliform loop as the sim- 

 plest type of loop in the Ancylobrachia. Besides Centronella,, 

 other adult representatives of the same structure are Rens- 

 selceria and Newherria. They are all late Silurian, Devonian, 

 and Carboniferous genera, but the centronelliform structure 

 continues later, and is represented in the Trias by the genera 

 Juvavella Bittner and Nucleatula (Zugmayer) Bittner. 



It was at once suggested that interesting results might be 

 obtained in studying the development of a spire-bearing 

 brachiopod, and, as the earliest species more clearly show 

 their phylogeny in their ontogeny, the ancient genus Zygos- 

 pira was selected. Very complete material was accessible, 

 collected by the writers, from the Trenton of Minnesota and 

 Kentucky, so that series of specimens were assembled repre- 

 senting all stages of growth from specimens .8 mm. in length 

 to mature size. They were prepared to show their brachial 

 supports, and it is clearly demonstrated that the i^rimitive 

 arm support in Zygospira is a terebratuloid loop having a 

 Centronella-\\\.Q form, which undergoes several modifications 

 before the growth of the spiral lamellae, thus in so far re- 

 sembling the development of Dielasma. 



These results threw doubt on a number of Lower and 

 Upper Silurian species described as having recurved loops, 

 and previously referred to the higher terebratuloid genera 

 Macandrevia or Waldheimia. The shells are impunctate, 

 while Mensselceria and Centronella are distinctly punctate, 

 like all other well-known Terebratulge. Upon investigation 

 it has been ascertained by Hall and Clarke and the writers 

 that the species which have been referred to Hallina and 

 Macandrevia from the Silurian are spire-bearing forms, and 

 therefore do not belong to the Ancylobrachia. 



Fischer and Q^hlert have called attention to a number of 

 recent species which have been erroneously based upon the 



