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 CapewelH 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. 



FIO. 236. ri(J.237. Fig! 538. Fio. 239. FiG. 240. 



EiehuxUdia CapeweM, Davidson. 

 Fig. 236. Hexagonal cells of the outer surface of tlie sltell in unworn specimens. 

 Fig. 2S7. Small polygonal cells in walls of hexagonal cells. 



Jig. 238. Polygonal cell layer between outer hexagonal cells and inner dense layer. 

 Fig. 238. Perforated inner dense layer. 



Fig. 240. Vertical section of the shell; a, outer hexagonal cell-walls; b, polygonal cell layer; c, inner dense layer 

 with minute perforations. (Youno ) 



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 laminae 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 



*8ee Bkhohbr and Clabkb, Memoirs N. Y. State Museum, vol. i, No. 1, p. 32. 



t Babrahdb, SystJme Silurian, vol. v, pi. Ixxxi, tigs. 1, 2j Beecubs and Clabke, op. cit, pi. iii, fig. 11. 



