CLASS II 



BRACHIOPODA 



359 



Fio, 529. 

 Cyrtina heteroclyta 



called a false cardinal area (Fig. 546, B, G). A split tubulär structure, or syrinxy 



whicli partially encloses tlie pedicle, is developed in the delthyrium of some spire- 



beariiig forms. 



The deltidium has its origiu in the Cephalula stage of Protremata {Thecidea 



mediterranea) contemporaneously with the rudiments of the dorsal 



and ventral valves, while the embryo is still in the free swimming 



condition. The dorsal valve and incipient deltidium appear first, 



being secreted by the rudimentary dorsal mantle and the dorsal sur- 



face of the body, the latter subsequently becoming the pedicle. The 



ventral valve is formed last, but is widely separated from the dorsal. 



Between the two valves is placed the short and thick, but as yet 



unattached pedicle, on the dorsal surface of which the third plate, 



known as the prodeltidium, still remains. Shortly before the animal 



becomes fixed by the pedicle, the prodeltidium fuses with the posterior (Defr.). High car- 

 -.1 iimi TT. ,. ^ , dinal arpa of ventral 



margin ot the ventral valve. The pedicle is at this stage entirely valve showing dei- 

 surrounded by shell, being enclosed on one side by the ventral valve, fuged'" deiTidia^ 

 and on the other by the deltidium. The latter plate then continues piates, or i)seudo- 

 to grow as one piece, extending from the apex in an anterior direction, 

 and is secreted entirely by the pedicle (Fig. 541). The deltidium is never punctate 

 in structure, but it may l)ear spines (Äulosteges), and sometimes exhibits a round er 

 pedicle Perforation {Glitambonites). The deltidium is characteristic of the Protremata, 

 while a similar plate, the homoeodeltidnim, is developed in some of the Neotremata 

 and Atremata. This covering of the delthyrium is always present in the young of the 

 Protremata, but is absent in the Telotremata. In many of the Protremata (Orthidae, 

 etc.) the deltidium is only present in nepionic stages of the individual, being 

 resorbed at maturity ; but in the great majority of these forms it remains persistent 

 throughout life. 



Deltidial -piates occur only in the order Telotremata, and consist of two pieces 

 which begin as narrow, linear, calcareous piates, growing medially from the walls of 

 the delthyrium, They gradually increase in size, and usually come in contact 

 medially with one another, either below or above the pedicle foramen, and are 

 secreted by an extension of the ventral mantle lobe. Thus in respect to their origin 

 they differ from the deltidium of the Protremata, which is secreted by the pedicle, 

 and not by the mantle. The deltidial piates are never present in the earliest growth 

 stages of the shell, the delthyrium being then an open triangulär fissure through 

 which the pedicle is protruded. In the adult stage the deltidial piates may remain 

 as narrow, linear discrete piates (Fig. 530, B) ; may meet beneath the pedicle foramen 



Fig. 530. 



A, Cyclothyris vespertilio (Brocchi), with united deltidial piates. B, Terebratella dorsata (Lam.), with 

 discrete deltidial piates. C, Young specimen of Stringoce2yhalus bnrtoni (Defr.), with the deltidial piates united 

 above the foramen. 



(Fig. 535, B) ; or they may wholly enclose the pedicle (Fig. 530, A). The deltidial 

 piates frequently unite l)y fusion, when they closely resemble a true deltidium, and 

 are known as a pseudodeltidium {Gyrtia, Cyrtina, Fig. 529). 



The chüidium is a convex plate which often covers the cardinal process of the 



