srHucHERT.] THE PRODELTIDIUM. 91 



_not yet kuown. It is evident, however, from the material Mr. Walcott 

 possesses, that Iphidea-like forms will be discovered in which the car- 

 dinal area is undeveloped and in harmony with the protegulum. 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 plate originating on the dorsal side of the body wall in the 

 cephalula stage of Tltecidium mediterraneum, the only living species of 

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

 has been observed bj^ authors as also occurring in the Atremata and 

 Neotremata. The term prodeltidium 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 order. That tliis plate is also present in the Neotremata is 

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

 Discinisca) given by Fritz Mueller. These larvai were captured in 

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

 so successful as Kovalevsky and otliers in securing the earlier larval 

 stages of other genera developing in 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 young 

 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 (Neotremata 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 Neofremata. Since it was first discovered bj^ Fritz Mueller 

 in 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:' 



Mit ibrem Hiuterraude dem ausgebucbteteu Hinterrande deuHBauchsliale anliegeud, 

 gewahrt man zwiscben den Scbalen eine queroraJe Platte, 0.06 mm. laiig, 0.11 bieit, 

 mit diinklerem, oft braunriitblich gefitrbtem, riiigfJirmigeii Raudo. Sie baftefc an 

 der Baucbscbale, deren Bewegungen sie folgt, und steht mit der Riickenscbale nur 

 durch Muskeln in Verbindung. 



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

 loosely attached to the ventral shell near its i^osterior margin, the 

 movements of which it follows. Mueller adds:'^ 



Die querovale Platte tritt untir de^bis zum Vorderrande der Riickenscbale vorge- 

 scbobenen Baucbscbale vor, beginnt sicb nacb binten zu verlangereu uud ein 

 faseriges Anselien zu zeigen (Stiel?); sie folgt, nacb wie vor, den Bewegungen der 

 Baucbscbale. 



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



