80 ScliAicJiert — On the Development c)f tJie 



o})en deltli_yriuin is graduall_y reduced in size by the introduction 

 of deltidial plates which grow inwardly from the walls of the 

 fissure, being wider anteriorly where they join, leaving in the 

 apex an oval pedicle opening. As the beak incurves these plates 

 become larger, stronger, and anchylosed along the median line, 

 but at maturity are nearly completelv liidden by the dorsal 

 uiiilx). The pedicle o];)ening at maturity (fig. 9) is through the 

 acutely convex portion of the ventral uml)o, and is compara- 

 tively smaller in size than during ])revious stages. 



Growth Stages.— A well-preserved specimen ofal)Out l.o mm. 

 length (fig. 10), shows three distinct stages of groAvth before the 

 introduction of the plicated or specific period : a, the initial shell 

 or protegulum, with l)oth cardinal lines arched ; I), a broad, oval 

 stage, in which the ventral liinge areas on each side of the delthy- 

 rium first appear, followed by c, a subcircular form, with tlie 

 beginning^ of the fold and sinus. It is either during stage c or !> 

 or both that the concave i)late in' the apex of the delthyrium is 

 develo})ed. During the next or fourth stage the first specific 

 characters begin to appear, as shown by the i)lications, and also 

 tlic first stage of the calcareous l)rachial supports. 



Otmerration^ mid Currelntions. 



The first or initial shell in Zygospira, as in other l)rachioi)ods' 

 is the ])rotegulum, which has been com2:)ared with adult Paterinn, 

 of the Lower Primordial. In many inarticulate brachiopods it 

 is known that the ])rotegidum is followed 1)}^ a nearly round 

 Oholella-like inarticulate stage, but in all rostrate articulate spe- 

 cies in which the second stage has been observed there appears 

 the fir.st articulation of tlie valves. The fold and sinus, along 

 with a few rudimentary plications, are introduced during the 

 third stage of Zygospira. This form of shell much resembles 

 some })rimordial species which have been })rovisionally referred 

 to CamareUa. W\i\\ Imt slight modifications in the convexity of 

 tlie valves and the greater or less prominence of the fold and 

 sinus, this form is repeated in a number of earh' Paleozoic genera 

 of the su1>orders Tnillacen and Rndraeed, as primordial (Jamarella 

 and many species of PentameruH^ Zygospira nicoletti, Caiiiarelhi 

 l)iscnlata, Dayin, and the so-called ]Vnld/ieimias of the Upper 

 Silurian. It is therefore impossible to refer with certainty on 

 the l)asis of external character alone any Lower Silurian brachi- 



