BRACHIOPODA. 
309 
this point its anterior edge is concave, the septum disappearing not far within 
the margin of the valve. 
No traces of muscular scars have been observed on either valve. 
The external surface of the valves is covered by a coarse network of super¬ 
ficial cells, usually hexagonal, sometimes circular in outline. In all species and 
in early growth-stages there is a bare, smooth, triangular area at the beak of the 
pedicle-valve, where this superficial ornament does not extend. 
It has been shown by Young that the shell of Eichwaldia CapeweM is composed 
of three layers; first, the outer, coarsely meshed and wholly superficial layer; 
below this, a more compact layer perforated by numerous small polygonal cells. 
m 
riG. 236. 
>0o0 
KlG.237. 
Fig. 238. 
Fig. 239. 
Eichwaldia Capewelli, Davidson. 
Fig. 236. Hexagonal cells of the outer surface of the shell in unworn specimens. 
Fig. 237. Small polygonal cells in walls of hexagonal cells. 
Fig. 238. Polygonal cell layer between outer hexagonal cells and inner dense layer. 
Fig. 239. Perforated inner dense layer. 
Fig. 240. Vertical section of the shell; a, outer hexagonal cell-walls; 6, polygonal cell layer; c, inner dense layer 
with minute perforations. {Young ) 
the apertures of which are exposed in the greater cells of the outer layer; and, 
on the inner surface, a dense lamina with minute perforations. 
The peculiar bare spot on the umbo of the pedicle-valve, from which the 
external shell-layers are absent, requires a brief notice. This area is the open¬ 
ing of an aperture entering the valve between the outer shell and the internal 
umbonal diaphragm. The smooth surface of the area is the inner surface of 
this diaphragm, which is considerably thickened about its apex. Young 
has called attention to the fact that along the margins of this bare spot the 
superficial laminm are unfinished and the edges of this outer layer rough and 
ragged. This is especially true of the anterior edge, while the lateral edges 
appear to be invariably straight and to diverge at a constant angle.* The lat¬ 
ter evidently represent the lines of attachment of the internal diaphragm to 
the lateral walls of the valve. In the youngest shells that have been observedf 
* See Beecher and Ouaeke, Memoirs N. Y. State Museum, vol. i, No. 1, p. 32. 
t Baeeande, Systeme Silurien, vol. v, pi. Ixxxi, figs. 1, 2; Beecher and Clarke, op. cit., pi. iii, fig. 11. 
