92 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. [uvll.SI. 



Since in this stage of Discinisca there is no pedicle present, Mueller 

 apparently was disposed to regard the ])rodeltidium as the equivalent 

 of the pedicle. That this is an erroneous interpretation seems certain, 

 for in his second pa])er he states: ' 



Die bis dahin zwisclieu den Sclialeii verhorgene querovale Platte (der Stiel) tritt 

 hervor, indein ^3io sich wie es scheint, mu dcm ausgebucbteu llinterrande des Baiich- 

 schale vollstiindig heiiimdreht und so ihr vorderer Rand zum hinteren wird. 



In Glottidia the pedicle does not appear until sometime after the 

 prodeltidium is developed, and it seems reasonable to assume from the 

 description of Mueller that, on the development of the pedicle, the 

 prodeltidium is pushed and turned backward, and between this and 

 the notched ventral margin the pedicle passes. The pedicle opening at 

 this stage is therefore surrounded by shell matter, anteriorly by the 

 protegulum and posteriorly by the prodeltidium, characters duplicated 

 in Thecidium. In the latter genus the prodeltidium develops into the 

 deltidium, whereas, according to Mueller, this plate subsequently dis- 

 appears in Discinisca. Brooks, also, is not disposed to accept Mueller's 

 interpretation of this plate as the pedicle, since he writes : ^ 



If it is tbe same [tbe transversely oval plate of Discinisca and tbe dorsal semicir- 

 cular plate of Glottidia], Mueller is certainly in error in bis suggestion tbat it is the 

 peduncle, for tbere is no connection between tbe two structures. 



In Glottidia jyyramidata, Brooks has shown that the prodeltidium is 

 also present, yet here it does not become attached to the ventral shell, 

 but is firmly fastened to the dorsal valve, and this apparently was 

 consummated in the paternia stage. Brooks writes: 



I was not able to learn anytbing of tbe significance of tbe semicircular plate 

 sbown in figures 1 and 3. It is found onlj'^ in tbe dorsal valve, and is either a mark 

 upon its inner surface or a plate between tbe body and tbe valve. According to 

 Fritz Mueller, tbe Bracbiopod larva studied by bini possessed a similar structure. 

 * * * Tbe embryo of Lingula is so suuill and tbin tbat if this were a separate 

 plate, it would be ratber difiicult to prove without seeing it move, or find it bent 

 outward. In tbe absence of such evidence, we seem warranted in condudiug tbat it 

 is a similar structure to the movable plates of Mueller's larva, although, in Lingula 

 at least, it is in connection with the dorsal, not the ventral valve. 



No one has yet mentioned the presence of the prodeltidium in living 

 Telotremata, and it may prove to be absent in this order, as it is not 

 developed in the three species carefully studied by Morse, Kovalevesky, 

 and Shipley. 



RccajntHlation. — The prodeltidium is present in Atremata, Neotre- 

 mata, and Protremata. In the embryonic brachiopods developing this 

 plate it is first found on the dorsal side of the body wall, and later is 

 anchylosed to the ventral shell in Protremata (Thecidium). In the 

 Neotremata, the earliest embryonic stages of which are not known, it 

 is found completely developed and loosely attached to the ventral shell, 

 anterior to the posterior margin. It subsequently turns backward to 



' Archiv fiir Naturgesch., 1861, p. 54- 



" Chesapeake Zoological Laboratory, session of 1878; Johns Hopkins University, 1879. 



