sPHicHEKT.] THE PRODELTIDIUM. 91 



Dot yet known. It is evident, however, from the material Mr. Walcott 

 possesses, that ri)hi(lea-like forms will be (liscovered in which the car- 

 dinal area is nndeveloped and in harmony with the prote^ulum. It is 

 in this sense that the terms Paterina and paterina stage are used 

 throughout this work. 



THE PRODELTIDIUM. 



The term prodeltidium is applied by Hall and Clarke to the third 

 shell i)late originating on the dorsal side of the body wall in the 

 cephaluhi stage of Thecidlum incditerraneum, the only living species of 

 Protremata. This plate, however, is not restricted to that order, but 

 has been observed by authors as also occurring in the Atremata and 

 Neotremata. The term i)r()deltidinm is here applied to this embryonic 

 plate wherever it occurs unmodified. 



Beecher has shown that the prodeltidium in the Protremata is the 

 first cause for the development of the deltidium so characteristic 

 of this Older. That this plate is also present in the N^eotremata is 

 apparent from the description of a brachiopod larva of Discina ( = 

 Discinisca) given by Fritz Mueller. These larvae were captured iu 

 abundance off Desterro or Santa Cathrina, Brazil, but Mueller was not 

 So successful as Kovalevsky and others in securing the earlier larval 

 stages of other genera developing iu the brood pouch, and therefore 

 nothing is known as to the place of origin of the prodeltidium in Neo- 

 tremata. Since, however, the prodeltidium is also present in youug 

 Lingula of the order Atremata, where it is wholly attached to the inte- 

 rior of the dorsal shell, it appears safe to assume that this plate inva- 

 riably develops on the dorsal side of the thoracic segment of embryonic 

 brachiopods, and later becomes attached either to the dorsal (Atremata) 

 or ventral valve (Xeotremata and Protremata), except where, as in the 

 Telotremata, it does not occur. 



Before taking up the phylogenetic significance of the prodeltidium, 

 it will be advisable to state what is known of this plate in the Atre- 

 mata and Neotremata. Since it was first discovered by Fritz Mueller 

 ill the Neotremata, where also it is best developed, and subsequently 

 was homologized by Brooks with a similar plate in Glottidia, it will 

 here be given first consideration. Mueller writes:' 



Mitilirem Hiiiterraiide dem ansgcbucliteten Hinterrandedes Banchshale anliegend, 

 gewahit man zwischen den Schalen eiue querorale Platte, O.Oti mm. laug, 0.11 breit. 

 mit diinkler«>rii, oft brannrilthlich getarbtem, ringturmigen Raude. Sie haftefc an 

 der Banchscbale, dereu Beuegungen sie folgt, und steht mit der KiickenscLale unr 

 diirch Miiskeln in Verbindung. 



There is, then, in this Discinisca, a transversely oval plate somewhat 

 loosely attached to the ventral shell near its posterior margin, the 

 movements of which it follows. Mueller adds:- 



Die (|iierovale Platte tritt ttntir des bis zum Vorderrande der Riiekcn.schale vorge- 

 scbobenen Baucbschale vor, beginnt sich nach binten zu veiiangereu und ein 

 fa«eriges Ansebeu zu zeigea (Stiel?); sie folgt, nach wie vor, den Bewegungen der 

 Baiieliscliale. 



' Archiv Anat., Physiol., 1860, p. 74. 2 Ibid., p. 78. 



