92 SYNOPSIS OF AMERICAN FOSSIL BRACHIOPODA. IudluS?. 



Since in this stage of Discinisca there is no i)e(li('le present, Mueller 

 apparently was disposed to regard the prodeltidinm as the equivalent 

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

 for in his second paper he states: ' 



Die bis dahiu zwischen den Schalen verborgene qiierovale Platte (der Stiel) tritt 

 hervor, indem sie sich Avie es sclieint, iim dem ausjjebnchteu Hiiiterrande des Bauch- 

 scbale vollstiindig herumdreht uiid so ihr vorderer Raud zum binteren wird. 



In Glottidia the pedicle does not appear until sometinae after the 

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

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

 prodeltidinm 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 prodeltidinm, characters duplicated 

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

 deltidium, whereas, according to Mueller, this i^late 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 the same [the transversely oval plate of Discinisca and the dorsal semicir- 

 cular plate of Glottidia], Mueller is certainly in error in his suggestion that it is the 

 peduncle, for there is no connection between the two structures. 



In Glottidia' pyramidata, Brooks has shown that the prodeltidinm 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 anything of the significance of the semicircular plate 

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

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

 Fritz Mueller, the Brachiopod Jarva studied by him possessed a similar structure. 

 * * * The embrj'O of Lingula is so small and thin that if this were a separate 

 plate, it would be rather dififlcult to prove without seeing it move, or find it bent 

 outward. In the absence of such evidence, we seem warranted in concluding that 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 prodeltidinm 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. 



Recapitulation. — The prodeltidinm 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 fvir Naturgesch., 1861, p. 54. 



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



