ANTHRACOMYA WILLIAMSONI, VAR. 103 
4*, AnTHRACOMYA WILLIAMSONI, var. opTuUsA, Ludwig. Plate XV, figs. 5—9. 
Unto ostusa, Ludwig. Paleontographica, vol. viii, p. 38, pl. iv, figs. 13—15, 
1859-60. 
AntHRACoMYA oBTusa, Hind. Quart. Journ. Geol. Soc., vol. xlix, p. 267, pl. x, 
figs. 14, 14a, 1893. 
Varietal Characters.—The shell is quadrately oblong. The anterior end is 
almost obsolete, constricted anterior to the umbonal fold, its border blunt and 
obtusely rounded. The dorsal and ventral margins are almost parallel, the inferior 
border straight. The posterior end is truncate, the upper and lower angles being 
bluntly rounded. The hinge-line is curved anteriorly as far as the umbo, but 
posteriorly is straight and slightly elevated. The umbones are almost anterior, not 
contiguous, small, hardly raised above the hinge-line. The posterior part of the 
shell is elongated and flattened.. The upper border of the posterior umbonal ridge 
is prolonged backwards and downwards, but very soon becomes lost on the surface 
of the shell; above this line the shell is compressed into the hinge-line. 
Interior normal. 
Exterior.—Surface-markings as in A. Williaimsoni. 
Dimensions : 
Antero-posteriorly. Dorso-ventrally. Laterally. 
Pl. XV, fig. 5, measures. . 25 mm. 12 mm. 8 mm. 
Locality.—With A. Williamsoni in the roof of the Hard Mine Coal of North 
Staffordshire. 
Observations.—This form occurs sparingly with specimens of A. Williamsoni, 
and only differs from that shell in the anterior position of the umbones and 
obsolete anterior end, and in consequence has a less conspicuous oblique ridge, and 
is generally less convex, and more flattened. The shell I show (PI. XV, fig. 5) 
agrees in every character with the figure and description of Unio obtusus, 
Ludwig. Seeing how the position of the umbo varies in many of the species of 
this genus, and that the varietal form is always found associated with 
A. Williamsoni as far as | am aware at present, [ have not thought it advisable to 
perpetuate Ludwig’s name as that of a specific form, but have adopted it as 
denoting a fairly constant and well-marked variety. 
The varietal is much rarer than the specific form, and I should estimate that it 
is not met with more frequently than in the proportion of 1 in 40. 
