BRACHIOPODA. 341 



already discussed. It appears ia a highly developed state in conjunction witli 

 the unmodified deltidiuin, first in Protorthis, of the Cambrian, then in 

 PoLYTCECHiA, Syntrophia, Clitambonites and ScENiDiUM, of the early and later 

 Silurian and of the Devonian. 



A parallel line of development is exhibited by spondylium-bearing forms in 

 which the deltidium disappeared at a very early period, and the shells possess 

 a trihedral, generally coarsely plicated and decidedly rhynchonelloid exterior. 

 It seems highly probable that this line was differentiated in the early Cambrian, 

 as indications of this structure are observable in some primordial species, as 

 Camarella? minor, 'SYaicoit, and Stricklandinia ? Balclekhensis, Davidson ; in the 

 Silurian it is represented by Camarella and Parastropiiia ; also by the more 

 rotund and more finely plicate shells, Anastrophia, Porambonites,Lycopiioria and 

 NoETLiNGiA. The last-named genera are not homogeneous with the others in 

 the phases of development which they represent, all of them retaining the 

 cardinal areas more or less distinctly, while Lycopiioria and Noetlixgia also 

 possess a cardinal process in the brachial valve. The presence of the cardinal 

 area in such early structures must be regarded as a retention, rather than a 

 resumption of a primitive character. 



Whatever may be the oscillation in form and the variation in secondary 

 characters presented by Camarella, Parastrophia and their allies, present evi- 

 dence indicates that they must be regarded as the genetic precursors, as they 

 are the secular precedents of the great group of true pentameroids (Pentamerus, 

 Capellinia, Conchidium, Barrandella, Sieberella, Pextamerella, Gypidula, 

 Stricklandinia, Amphigenia); and, indeed, the last of these pentameroids, Cam- 

 arophoria, of the Carboniferous and Permian faunas, is an exemplification of, 

 and in fact a return to the rhynchonelloid exterior and the camarellid aspect, 

 with the addition of deltaria in the delthyrium. 



While considering in detail the pentameroid genera mentioned above, it has 

 been shown that in certain of them, as Pentamerus and Conchidium, a true 

 deltidium is often retained, though it is a fragile structure rendered concave by 

 the arched growth of the umbones of the valves, and is generally absent. In 

 others, as Gypidula and Pentamerella, there are occasionally evidences of lat- 



