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. 9 a, PI. 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 

 to erosion. 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, PI. 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 iridinoides, 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*. Antheacomya Adamsii, var. expansa, Hind. Plate XIII, figs. 1 — 3. 



Anthracomya Adamsii, Ward. Trans. N. Staft". Inst. Min. and Mech. Engin., 



vol. x, 1890, p. 125, pi. i, fig. 2. 

 — — var. expansa, Hind. Quart. Journ. Geol. Soc, vol. xlix, 



1893, p. 2G1, pi. 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. 



