80 Schmhert On the Development of the 



open delthyrium is gradually 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 completely hidden by the dorsal 

 ambo. The pedicle opening at maturity (fig. 9) is through the 

 acutely convex portion of the ventral umbo, and is compara- 

 tively smaller in size than during previous stages. 



Growth Stages. A well-preserved specimen of about 1.5 mm. 

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

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

 or protegulum, with both cardinal lines arched ; 6, a broad, oval 

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

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

 beginning of the fold and sinus. It is either during stage c or b 

 or both that the concave plate in the apex of the delthyrium is 

 developed. During the next or fourth stage the first specific 

 characters begin to appear, as shown by the plications, and also 

 the first stage of the calcareous brachial supports. 



Observations and Correlations. 



The first or initial shell in Zygospira, as in other brachiopods' 

 is the protegulum, which has been compared with adult Potcrwa 

 of the Lower Primordial. In many inarticulate brachiopods it 

 is known that the protegulum is followed by a nearly round 

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

 cies in which the second stage has been observed there appears 

 the first articulation of the 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 primordial species which have been provisionally referred 

 to Camarella. With hut slight modifications in the convexitv of 

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

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

 of the; suborders Trulftura and Rostracca, as primordial ('<im<itrlln 

 and many species of /'cntamerus, Zyf/o*i>iru iiicolcfti, ( 1 <nit<tr//i<i 

 hixr.iihitn. Dnij'm. and the so-called \\'<tl<lheimiax of the Upper 

 Silurian. It is therefore impossible .to refer with certainty on 

 the basis of external character alone any Lower Silurian brachi- 



