BRACHIOPODA. 
53 
faunas of still earlier date. Rhinobolus has the aspect of a degenerate Trim- 
erellid, marked by the general obsolescence of the characteristic features of the 
group. In Lakhmina, we have evidence that the platforms attained a con¬ 
siderable development before the introduction of Silurian faunas. Features, 
which in American faunas appear to have developed slowly, and whose different 
stages can be satisfactorily traced, seem to have been specialized with great 
rapidity in this obscure fossil. It is not necessary to assume that this fossil 
from a distant fauna, now regarded as of primordial age,* will materially 
modify the conclusions expressed in the foregoing diagram, derived from the 
study of American faunas. 
Having indicated that the inception of the platform was probably due to aug¬ 
mented muscular energy and concomitant increased secretion of muscular 
fulcra, we may revert to the consideration of the function of this organ in its 
highest development, and to the inquiry, whether this attainment may have 
been acquired alone by the constant action of the forces named, or has been 
aided to this result by the accessory action of other organs. 
Whether or not the platform be hollow as in Trimerella and Lakhmina, and 
sometimes in Dinobolus, or solid, as usually in Dinobolus, and always in the 
other genera, there is invariably a manifest tendency toward its excavation. The 
anterior walls are always concave; in Monomerella the concavity of the plate 
is deep, the anterior moiety being turned with a steep slope toward the cavity 
of the shell; furthermore, the shell itself is much depressed beneath this 
projecting wall and on either side of the anterior median septum into which 
the platform is continued. The same features are seen in the brachial valve 
throughout these genera.f Even in Rhinobolus, all that is left of the platform 
is turned upward and inward at this sharp angle. In the pedicle-valve, how¬ 
ever, this inclination of the platform, the excavation of its anterior walls and 
* See page 29. 
f Mr. Ulrich describes a peculiar structure in the platform of Lingulasma. He says: “ The cast of the 
interior which furnished the gutta-percha squeezes represented by figs. 5 and 5 a” (see PI. II, fig. 19), “orig¬ 
inally preserved much of the shell and all of that pertaining to the platform. This was carefully removed, 
and during the process it was noticed that the platform consisted of numerous cup-shaped laminae placed 
within one another, and so that an open space was left between each and the preceding and succeeding ones ” 
(American Geologist, vol. iii, No. 6, p. 386. 1889). 
