72 Beecher an-l Schuchert Development of the 



from the St. Louis group of the Lower Carboniferous in Ken- 

 tucky. The shells are partially silieified, generally filled with 

 transparent calcite, and afford very satisfactory preparations of 

 the arm supports. It was found that the loop of Dlelasma un- 

 derwent transformations during growth, and that the earliest 

 stage observed is like Centronella. This establishes the centro- 

 nelliform loop as the simplest type of loop in the Ancylobrachia. 

 Besides Centrondla, other adult representatives of the same 

 structure are Renssd&ria and Newberria. They are all late Silu- 

 rian, Devonian, and Carboniferous genera, but the certfrorieUiform 

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

 genera Javavella Bittner and Nudeattda (Zugmayer) Bittner. 



It was at once suggested that interesting results would be ob- 

 tained in studying the development of a spire-bearing brachio- 

 pod, and, as the earliest species more clearly show their 

 phylogeny in their ontogeny, the ancient genus Zygo*i>mt was 

 selected. Very complete material was accessible, collected b} r 

 the writers from the Trenton of Minnesota and Kentucky, so 

 that series of specimens were assembled representing aU 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 primitive arm support in Zygoxjtira 

 is a terebratuloid loop having a Centronella-like form, which un- 

 dergoes several modifications before the growth of the spiral 

 lamella?, and thus in so far resembling the development of 

 Didasma. 



These results threw doubt on a number of Lower and Upper 

 Silurian species described as having recurved loops and pre- 

 viously referred to the higher terebratuloid genera Mawuidirrta 

 or Waldhei.nua. The shells are impunctate, while Uenxxchcriu. 

 and Centronella are distinctly punctate, like all other well-known 

 terebratulao. Upon investigation, it has been ascertained by 

 Hall and Clarke and the authors that the species which have 

 been referred to Hallina and Macandrcriu from the Silurian are 

 spire-bearing forms, and therefore do not belong to the Ancylo- 

 brachia. 



Fischer and (Ehlert have called attention to a number of 

 recent species which have been erroneously based upon the 

 immature stages of higher species, and in the Terebratellida 1 it 

 is evident that great uncertainty must exist in the identification 

 ot specimens not fully adult. Now, finding that Paleozoic genera 



