Brachiopoda. 55 



gradually fills up the triangular fissure till only a small foramen GALLERY 

 is left at its apex, as in Clitamhonites (I'ig. 98) and Rafinesquina ^^^^ g/^^ 

 (Diagram 4) ; later in life the pseudo-deltidium may be reabsorbed, wall-cases 



10 & 11, 

 Table- 

 cases 

 86-88. 



Fig. 98. — Clitambonites Verneuili, Eichw,, sp. Ordovician. 

 A.U example of the Protremata. 



The shell is seen from the side of the brachial valve, above which is the lofty 

 hinge-area of the peduncle valve. The delthyrium is covered by a single 

 pseudo-deltidiura, through which the peduncle passed by the foramen (K). 



as in the Orthidae : such genera constitute the order Proteemata. 

 Some forms have taken another line of evolution : in them the 

 pseudo-deltidium is either absent or soon reabsorbed, so that the 

 delthyrium is open in early life, but at a later period it becomes 

 partly or entirely closed by tv?o **deltidial plates," which are 

 secreted by the edges of the mantle along the sides of the 

 delthyrium, and which may subsequently meet either above or 

 below the peduncle, and may even fuse into one plate, the 

 * * deltidium " ; these plates are well seen in Alrypa (Fig. 95) and 

 String ocephalus, and occur in Rhynchonella, Spirifer (Fig. lol), 

 Terehratula, and allied forms : such genera constitute the order 

 Telotkemata. 



The orders Atremata and Neotremata are equivalent to suc- 

 cessive stages of the Inarticulata, and are most abundant in the 

 earlier Palaeozoic rocks. From them the Protremata and Telo- 

 tremata arise as divergent groups, which together are the 

 equivalent of the Articulata, The Protremata were dominant 

 in later Palaeozoic time ; the Telotremata in Mesozoic. 



The shell-valves are secreted by the two mantle-folds which 

 line them. These are extensions of the body- walls, and they 

 contain prolongations of the body-cavity, in which is a blood-like 

 fluid and in which the generative products are formed. These 

 vessels often produce impressions on the inside of the shell, and 

 60 can be traced in the fossils, e.g. Camarophoria (Fig. 99) and 

 a fine specimen of Orthis [Schizophoria) striatula in the wall-case. 



