922 Report oF THE State GEoLoaist. 
cuta, Synrropata, CriramBonires and Sceniprum 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, Walcott, and Stricklandinia ? 
Balcletchensis, Davidson; in the Silurian it is represented by 
Camaretia and Parasrr para, by the more rotund and more 
finely plicate shells, Awastropara, Poramsonires, Lycopsorra and 
Nertuaia. 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 
Lyoornorra and Nazrrreta 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 resump- 
tion of a primitive character. 
Whatever may be the oscillation in form and the variation in 
secondary characters presented by CamarELia, ParasTROpHIA 
and their allies, present evidence indicates that they must be 
regarded as the genetic precursors, as they are the secular prece- 
dents of the great group of true pentameroids (Pent: merrvs, 
Caretta, Concaipium, BarraNpDELLA, SIEBERELLS, PENTAME- 
RELLA, Gyp puLA, StRIcKLANDINI4, AMPHIGENTA); and, indeed the 
last of these pentameroids, Camaropnorta, of the Carboniferous 
and Permian faunas, isan 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 Penramerus 
and Concuiprom, a true deltidium is often retained, though it isa 
fragile structure rendered concave by the arched growth of the 
umbones of the valves, and is generally absent. In others, as 
GypipuLa and PenraMEreELta, there are occasionally evidences of 
lateral, erect or convex growths upon the margins of the del- 
thyrium, which may be interpreted either as remnants of a 
resorbed convex deltidium, or as highly accelerated secondary 
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