BRACHIOPODA. 



113 



The relations of the peduncle to the valves manifest a ^jj?"^ 

 gradual evolution. The simplest type is seen in Iphidea wall-ease 

 (from the Cambrian, Fig. 58, a) and Lingula (Cambrian to 10 &d 11. 

 present day, Fig. 57), where the peduncle simply passes out 

 between the valves and is not enclosed by either of them ; 

 such genera are therefore without a hole (treina) through 

 which the peduncle may pass, and constitute the Order 

 Atremata. In Trematis, Orhiculoidea (Fig. 58, h), Siiohono- 

 treta, and their relatives (mostly Ordovician and Silurian), 

 the peduncle is restricted to the ventral valve ; it lies in a 

 groove or fissure (delthyrium), which remains open in primi- 

 tive forms, but closes round the peduncle (forming a trema) 

 in later forms : such genera constitute the Order Neotre- 

 mata. Next, the fissure in which the peduncle lies assumes 

 a triangular shape ; the peduncle is towards the apex of the 

 triangle, and itself secretes a single shelly plate (pseudo- 

 deltidium), which gradually fills up the triangular fissure 

 till only a small foramen is left at its apex, as in Clitainbonites 

 (Fig. 58, c) and Bafinesquina (Diagram 4) ; later in life the 

 pseudo-deltidium may be reabsorbed, as in the Orthidae : 

 such genera constitute the Order Protremata. Some forms 

 have taken another line of evolution : in tliem 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 two " 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 Atinjpa and String ocephalus, and occur in Bhynchonella, 

 Spirifer (Fig. 59), Terehrahda, and allied forms : such genera 

 constitute the Order Telotremata. The arms are often 

 supported by a calcareous skeleton, the shape of which is of 

 great importance in classifying fossil brachiopods. Thus, 

 the Telotremata branched into loop-bearing forms (Diagram 9 

 and Fig. 60, Magellania) and spire-bearing forms (Diagram 9, 

 Atrypa, and Fig. 59, Spirifer). 



The Orders Atremata and Neotremata are equivalent to 

 successive stages of the Inarticulata, and are most abundant 

 in the earlier Palseozoic rocks. From them the Protremata 

 and Telotremata arise as divergent groups, which together 

 are the equivalent of the Articulata. The Protremata were 

 dominant in later Palseozoic time ; the Telotremata in 

 Mesozoic. 



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