212 
a very decided angle at its j unction with the regularly curved ventral margin. 
The external surface is entirely covered with concentric folds of growth, 
which gradually become thinner as they aj)proach the anterior side of the 
valves. The test must have been extremely thin, as is proved by some traces 
of it which have been preserved, and by the slight difference between the 
ornamentation of internal casts and that of external impressions. None of 
the casts I have examined have preserved any trace of either the muscular 
impressions or of the pallial sinus ; but most of them have an empty space 
under the beaks that may have been occupied by the oblique plates of 
cartilage which characterise the genus. Occasionally some slight traces of 
this cartilage are preserved, thus leaving little doubt about the generic 
determination. 
Dimensions . — Some specimens reach a length of ten and a half, a 
breadth of eight and a half, and a thickness of about seven and a half, 
centimetres. 
Relations and Differences . — A superficial examination of this species 
would easily allow one to confound it with some varieties of Rachydomus 
globosus, J. I). Sowerhy, a description of which is given further on. It 
differs, however, not only in form, and in the regularity of its folds, but also 
in the thinness of its test, by the conformation of its hinge, and by the 
complete absence of all trace of muscular impressions on the internal casts 
of its valves. 
Horizon and Localities . — All these specimens were found in a rather 
friable, greyish sandstone, between Muree and Morpeth. 
Edmondia iNTEiiMEDiA, L. G. dc Koniiwk. 
PI. XVIII, Pig. L 
This is a rather large, moderately thick, suhtrigonal shell, a little 
longer than broad. Its anterior margin is convex ; its beaks thick and 
recurved, and placed towards the anterior third of the hinge line. The 
external surface is ornamented by regularly spaced concentric furrows, 
between which are numerous, and not very apparent, parallel striae. As in 
the preceding species the shell was very thin, and has entirely disappeared. 
Dimensions . — The length is nine and a half, the breadth seven and a 
half, and the thickness five and a half centimetres. 
[PI. 20, fig. 4.— W.S.D.] 
