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 prodeltidiwm, still remains. Shortly before the 
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 L; 
from the deltidium Fic. 493. 
of the Protremata, A, Rhynchonella vespertilio, with united deltidial plates. B, Terebratella dorsata, 
mae : with discrete deltidial plates. C, Young specimen of Stringocephalus Burtoni, with 
which is secreted by 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. 4938, A). The deltidial plates frequently unite, when they 
closely resemble a true deltidium (Cyrtia, Cyrtina, Fig. 492); they are then termed a 
pseudodeltidiwm. 
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 
Clitambonitidae and Strophomenidae (Fig. 538, 6). 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 origin and phyletie signi- 
ficance are therefore very different from those of the deltidium. 
The listrivm 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 Shell: 4rticulation—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 
