98 DEVONIAN FAUNA. 
femarks.—My specimens enable me to add something to the description given 
by Davidson. 
The foramen is minute and situate at the extreme apical point. Davidson 
states that it is concealed by the incurvature of the beak over it; this incurvature 
is not shown in my specimens, and therefore must have been extremely slight, or 
due to accident. On the other hand, the deltidium appears to have been large. 
The ventral valve bears a large thin median septum extending at least one- 
half down the shell, to which two large oblique dental plates converge. This is 
much like the arrangement in Pentamerus or Merista, but it evidently belongs 
to neither of these two genera, and I have not yet been able to find a true resting- 
place for it. In the dorsal valve there is also a thin median line extending more 
than halfway down; fragments of the shell being removed in two specimens show 
that this median line is, at least near the hinge-line, not a single septum, but the 
junction of two meeting at a small angle. 
The surface of the shell is smooth and dense without punctations, but with a 
few very minute, transverse, angular elevations. When the external surface is 
gone the structure of the shell takes the form of diverging radiating angular 
rods, which leave their mark on the cast. 
The peculiar marginal inward curve or “ wide, flattened, bi-concave border ” is 
not always seen. While this is greatly developed in most specimens, both large 
and small, three of my specimens show no signs of it, but their valves are regularly 
curved throughout, and meet at a very small angle. J am in some doubt whether 
these unthickened specimens ought not to be regarded as distinct, though it appears 
probable that they may be only immature though large examples. Mons. (hlert 
considers them as separable, and if so I would propose for them the provisional 
name of H. Cthlerin (Pl. XI, figs. 8, 8 a, 8 b). 
The shell varies very greatly in transverseness, as the following measurements 
of two of my specimens show :—Length 40 mm., width 35 mm., depth 25 mm. ; 
length about 53 mm., width 77 mm., depth 38 mm. 
IIL. Family.—Srrrireripa, King, 1846. 
1. Genus.—Merista, Suess, 1857. 
1. Murista pLEBEIA, Sowerby. Pl. XII, figs. 3—6. 
1840. Arrypa PLEBEIA, Sowerby. Geol. Trans., ser. 2, vol. v, pt. 3, pl. lvi, figs. 
12, 13. 
1864. Merista PLEBEIA, Davidson. Brit. Foss. Brach., vol. iii, pt. 6, p. 20, pl. iii, 
figs. 2—10. 
