CLASS II 



BRACHIOPODA 



359 



Fi(i. 529. 



Cyrtina hHeroclyta 

 (Defr.). High car- 

 dinal area of ventral 

 valve showing del- 



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

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

 bearing forms. 



The deltidium has its origin 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 jDrodeltidium fuses with the posterior 

 margin of the ventral valve. The pedicle is at this stage entirely 

 surrounded by shell, being enclosed on one side by the ventral valve, fl^iseci™ deTudia^ 

 and on the other by the deltidium. The latter plate then continues plates, or pseudo- 



, ■ j_ T r ii ■ J. • -I- I.- deltidium. 



to grow as one piece, extending irom tlie apex in an anterior direction, 

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

 in structure, Imt it may bear spines (Aulosteges), and sometimes exhibits a round or 

 pedicle perforation {Clitambonites). Tlie deltidium is characteristic of the Protremata, 

 while a similar plate, the hornccodfdtidium, 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 plates occur only in the order Telotremata, and consist of two pieces 

 which begin as narrow, linear, calcareous plates, 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 jiedicle, 

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

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

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

 as narrow, linear discrete plates (Fig. 530, i?) ; may meet beneath the pedicle foramen 



Fig. 530. 



A, Vyclothyris ves2oertilio (Brocchi), with united deltidial plates. B, Terebratella doraata (Lam.), with 

 discrete deltidial plates. C, Young specimen oi Stringocephalus burtoni (Defr.), with the deltidial plates united 

 above the foramen. 



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

 plates frequently unite by fusion, when they closely resemble a true deltidium, and 

 are known as a fseudodeltidium {Cyrtia, Cyrtina, Fig. 529). 



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



