BRACHIOPODA. 
341 
already discussed. It appears in a highly developed state in conjunction vith 
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 deltidiuin 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, Walcott, and Stricklandinia ? Baldetchensis, 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 Noetlingia 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, B.arrandella, Sieberella, Pentamerella, Gypidula, 
Stricklandinia, Amphigenia) ; and, indeed, the last of these pentameroids, Cam- 
arophoria, of the Carboniferous and Permian faunas, is an exemplification of, 
and in fiict 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- 
