BEACHIOPODA. 
113 
Tlie relations of the peduncle to the valves manifest a 
gradual evolution. The simplest type is seen in Iphidea 
(from the Cambrian, Fig. 58, a) and Lingula (Cambrian to 
present day. Fig. 57), where the peduncle simply passes out 
between the valves and is not enclosed by either of tliem ; 
such genei-a are therefore without a hole (trema) through 
which the peduncle may pass, and constitute the Order 
Atremata. In Trematis, Orhicidoidm (Fig. 58, h), Siphono- 
trcta, and their relatives (mostly Ordovician and Silurian), 
the peduncle is restricted to the ventral valve ; it lies in a 
gi’oove 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 iteelf 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 Clitamhonites 
(Fig. 58, c) and RaJinescjuina (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 them the pseudo- 
delthlium is either absent or soon reabsorbed, so that the 
deltliyrium 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 “ deltidiuni ” ; these plates are well seen 
in Atrgpa and Stringocephalus, and occur in Ehynchondla, 
Spirifcr (Fig. 59), Terehratula, and allied forms : such genera 
constitute the Order Telotrenaata. 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, 
Atrgpa, and Fig. 59, Spirifcr). 
Tlie Orders Atremata and Neotremata are equivalent to 
successive stages of the Inarticulata, and are most abundant 
in the earlier Palaeozoic 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 Paheozoic time ; the Telotremata in 
Mesozoic. 
I 
Gallery 
VIII. 
Wall-cases 
10 & 11 . 
