LAMPSIIELLS, ETC. . 



651 



Fm. 7.— The An- 

 cemtok of the 

 Braohiopoda 

 (Puterlna). Eii- 



lines of growth rim parallel to the curved margin of the shell, and up to the 

 hinge-line, whence we see that at all stages of its growth the 

 shell was of the same shape. A very large number of the 

 genera of the Brachiopods pass through a stage in early 

 youth in which both valves, or, more generally, the dorsal 

 valve alone, are of the same shape as the adult Paterina. 

 But during growth, changes in the shape of the valves 

 take place, chiefly produced by the enclosure of the 

 peduncle in the walls of the ventral valve, and by the 

 formation of a definite hinge. In the simple types of 

 Brachiopoda, such as Paterina and Llnrjula (Fig. 5), the 



peduncle passes out freely between both of the valves ; such forms are known 



as ATKiiHATA. 



Next, the peduncle-opening is restricted to the ventral valve, where it lies 

 in a slit, which may become transformed into a round opening by growth of sliell 

 substance, a stage that is still seen in the modern DtsciiMt and LHt:ciniiica (Fig. 

 8). Such forms are known as Neotkemata. The two orders, Atremata and 

 Ncotremata, are equivalent to the old order, Inarticulata or Trdeiitcnda. 



In the next order, the Pkotremata, the peduncle lies at the apex of a trian- 

 gular fissure (deltliyrium). This is partially closed in early life by a single 

 shelly plate (psoudo-deltidium), as in Clitamboiiites (Fig. 9). This plate is 

 secreted by the peduncle and posterior part of the body itself, and lies 

 towards the apex of the delthyrium. In the adult, the pseudo-delthyrium 

 becomes fused to the ventral valve, and often decreases in size. 



The last order is the Telotuemata, in which the delthyrium becomes closed 

 by the development of a pair of shelly plates (deltidial plates). These are 



D. V 



Firj. S.—Dlsctniscd. D., dorsal, and 

 /''., ventral, valve ; /',, peduncle 

 slit. Natural size. 



Fig 0. — Clitnmb'inilcsddseen- 

 dens. F. , foramen for 

 peduncle ; />., deltidium 

 of ventral valve ; C. , delti- 

 dium of dorsal valve 

 (cliilidium) ; V.A. and 

 D..^!., areas of ventral and 

 dorsal valves. Katural 

 size. 



secreted by the edges of the mantle, and may fuse, either in the adult or 

 from their earliest appearance, to form a single plate (deltidium). (See Fig. 10.) 

 Some of the earliest forms of this order are the Rhy)u:ho}idlidiV^ which occur 

 early in palaeozoic times. Subsequently, we find two main branches of the order, 

 in which the lo]jhophore is borne either by simple loops of shelly substance 

 (e.f/., TcrebraUda and Ma(fcllania\ or by coiled spires {e.g., Spirifer and 

 Atrypa) respectively (Fii^. il). The two orders Protnmata and Tdvtremata 

 together correspond to the old order, Articidata or Cll^tcntcrata, 



