72 BeecJicr ruid Schnj-lirrt — Drvclupiacnf of the 



from the St. Louis uroup oC the Lower (Jarltoiiiierous in Ken- 

 tuck 3^ The yhelli? are i)artially siliciHcd, generally tilled with 

 transparent calcite, and afl[brd very satisfactory preparations of 

 the arm supports. It was found that the loop of /)/^'/rf.s'»(a un- 

 derwent transformations during growth, and that the earliest 

 stage ohserved is like Centronella. This estaljlishes the centro- 

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

 Besides Centronella, other adult rei)resentatives of the same 

 structure are Rensselseria and Newh'errift. They are all late Silu- 

 rian, Devonian, and Carboniferous genera, but the cenironeUiform 

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

 genera Javavella Bittner and Niideatula (Zugma3'^er) Bittner. 



It was at once suggested that interesting results would be ol)- 

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

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

 ph3dogeny in their ontogeny, the ancient genus Zi/c/ospira was 

 selected. Verv complete material was accessible, collected l)y 

 the writers from the Trenton of Minnesota and Kentuckv, so 

 that series of specimens were assembled rei)resenting all stages 

 of growth from specimens .8 mm. in length to mature size- 

 The\" were prepared to show their l)rachial su])i)orts, and it is 

 clearly demonstrated that the [)rimitive arm support in Zijc/oxpira 

 is a terel)ratuloid looj) having a Ceiitronelki-like form, which un- 

 dergoes several modifications l)efore the growth of the spiral 

 lamellic, and thus in so far resembling the development of 

 Diclnsma. 



These results threw doul)t on a number of Lower and Upper 

 Silurian species described as having recurved loops and pre- 

 viously referred to the higher terebratuloid genera Macandrevia 

 or Widdlielmla. The shells are impunctate, while Rouselxria 

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

 terebratulffi. Upon investigation, it has l)een ascertained l\v 

 Hall and Clarke and the authors that the species which have 

 been referred to HaUina and Macandrevia from the Silurian are 

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

 brachia. 



Fischer and (Ehlert have called attention to a numl)er of 

 recent species which have been erroneously based upon the 

 inniiature stages of higher species, and in the Terel)ratellida' it 

 is evident that great uncertainty must exist in the identification 

 ot s})ecimens not fully adult. Now, finding that Paleozoic genera 



