LAMPSHELLS, ETC. 



651 



Fig. T.-THE AN- 

 CKSTOK OF THE 

 BRA.CHIOPOIM. 

 (Paterina). En- 

 larged. 



lines of growth run 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 Lingula (Fig. 5), the 

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

 as ATREMATA. 



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 shell 

 substance, a stage that is still seen in the modern Discing and Discinisca (Fig. 

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

 Neotremata, are equivalent to the old order, Inarticulate or Tretenterata. 



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

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

 shelly plate (pseudo-deltidium), as in Clitambonites (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 TELOTREMATA, in which the delthyrium becomes closed 

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



Fig 9. Clitambonites ddscan- 

 dens - ' dr * or 



Fifj. S.-Disciniscd. D. , dorsal, and 



^^atSS''^^ 0016 



sht. Natiualsize. dium of dorgal 



(chilidium) ; V.A. and 

 D. A . , areas of ven tral an d 

 dorsal valves. Natural 

 size. 



secreted by tho 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 EhynchonettidcR, which occur 

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

 in which the lophophore is borne either by simple loops of shelly substance 

 (e.{/., Tcrebratula and Magdlania\ or by coiled spires (e.g., Spirifer and 

 Atrypa) respectively (Fig. ll). The two orders Prolremata and Telotremata 

 together correspond to the old order, Articulata or Clistcntcrata. 



