CLASS ii BRACHIOPODA 295 



deltidium appear first, being secreted by the rudimentary dorsal mantle and the dorsal 

 surface 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 tin- 

 animal becomes fixed by the pedicle, the prodeltidium 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, and on the other by the deltidium. The 

 latter plate then continues to grow as one piece, extending from the apex in an 

 anterior direction, and is secreted entirely by the pedicle (Fig. 504). The del- 

 tidium is never punctate in structure, but it may bear spines (Aulosteges), and some- 

 times exhibits a round or pedicle perforation (Clitambonites). The deltidium is 

 characteristic of the Protremata, but it is also developed in some of the Neotremata 

 (Acrotretacea). 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. In numerous Telotremata, the deltidial plates unite medially at 

 maturity, when they may resemble a true deltidium. 



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 exten- 

 sion of the ventral 

 mantle lobe. In this 

 respect they differ 



from the deltidium FIG. 493. 



of the Protremata, A, Rhynchonella vespertilio, with united deltidial plates. B, Terebratella dorsata, 

 irTiifTi ia 00 rraf rl Tvir with discrete deltidial plates. C, Young specimen of Stringocephalus Burtoni, with 

 L D 7 the deltidial plates united above the foramen. 



the pedicle. 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. 

 493, B) ; may meet beneath the pedicle foramen (Fig. 498) ; or they may wholly 

 enclose the pedicle (Fig. 493, A). The deltidial plates frequently unite, when they 

 closely resemble a true deltidium (Cyrtia, Cyrtina, Fig. 492) ; they are then termed a 

 pseudodeltidium. 



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

 dorsal valve in the Protremata. It is particularly well developed in the families 

 Clitambonititlae and Strophomenidae (Fig. 538, B). The chilidium is not to be homologised 

 with the deltidium, since it never appears earlier than the adolescent stage, and is 

 apparently a secretion of the dorsal mantle lobe. Both its prigin and phyletic signi- 

 ficance are therefore very different from those of the deltidium. 



The listrium is a plate closing the progressive track of the pedicle opening or 

 pedicle cleft, in some Neotremata, posterior to the apex of the ventral valve. 



Internal Characters of the SheU : Articulation. The two valves are 

 held in apposition either by muscles only (Atremata and Neotremata), or they 

 are united by articulation (Protremata and Telotremata}. In the latter case 

 there are to be seen in the ventral valve a pair of cuneate or tooth-shaped 



