ANTHRACOMYA ADAMSII; Var. 91 
know of one specimen from the Horsleywood seam of the Northumberland Coal- 
field. I have not met with the form in European collections. It is limited to one 
horizon both in South Wales and North Staffordshire. The shells are usually in 
a good state of preservation, and have both valves attached, showing that they 
are in the place where they lived. They evidently had no byssus, as they are 
never attached to each other or to drift-wood, and they lie chiefly parallel to 
the bedding planes. 
The smallness of the external ligament is very remarkable, the whole of it, as 
shown in fig. 9a, Pl. XII, only measuring 12 mm., the length of the whole hinge- 
line being 57 mm. Posterior to the ligament the valves came in contact, much 
thinned and expanded upwards. This part of the shell is very delicate, and is not 
often seen, owing to the difficulty of extraction from a very hard matrix. 
The shell was exceedingly thin all over, and in many of the figured specimens 
has disappeared from the umbones ; but this is accidental, and not in any sense due 
toerosion. It would appear from the longitudinal grooves on either side of the cast 
of the hinge-line that in front and for about half the length of the hinge-line 
posterior to the umbones the edge of the shell was produced downwards or 
turned on itself at a right angle to form a hinge-plate, but this must have been 
absent at the posterior end; and it will be noticed in fig. 4, Pl. XII, that the 
grooves gradually become obsolete as they pass backwards. It is probable 
that there is a long posterior lateral tooth in that part of the hinge-line which 
occupied the grooves. This gradual thinning away of the hinge-plate obtains 
also in the other species of this genus in which I have been able to procure 
casts, and is somewhat analogous to the formation of the hinge-plate in San- 
guinolites ividinoides, M‘Coy. 
In addition to specimens having a comparatively elongate shape, there is a form 
of A. Adamsii which is of such constant occurrence that I considered it well in my 
paper in the ‘ Quart. Journ. Geol. Soc.’ to give it a varietal name. I still adhere 
to this opinion, though a series of intermediate forms are to be found. This 
form was figured by Mr. Ward (op. supra cit.) under the name of A. Adamsii, but 
the shape and position of the umbones differ considerably from those of the type. 
1*, Anruracomya ADAMsII, var. ExPANSA, Hind. Plate XIII, figs. 1—3. 
Anruracomya Apamsit, Ward. Trans. N. Staff. Inst. Min. and Mech. Engin., 
vol. x, 1890, p. 125, pl. i, fig. 2. 
-- — var. EXPANSA, Hind. Quart. Journ. Geol. Soc., vol. xlix, 
1893, p. 261, pl. ix, figs. 2, 3. 


think this is a mistake, as I only know this form from the Burnwood ironstone seam, in which bed, 
however, the other two species are absent. 
