ANTHRACOMYA WILLIAMSONI, VAR. 103 



4*. Anthracomya Williamsoni, var. obtusa, Ludwig. Plate XV, figs. 5 — 9. 



Cnio obtusa, Ludwig. Palaeontographica, vol. viii, p. 33, pi. iv, figs. 13 — 15, 



1S59-60. 

 Anthracomya obtusa, Hind. Quart. Journ. Geol. Soc, vol. xlix, p. 2G7, pi. x, 



figs. 14, 14 a, 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. Williamsoni. 



Dimensions : 



Antero-posteriorly. Dorso-ventrally. Laterally. 

 PI. 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. WHliamsoni i 

 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 I am aware at present, I 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. 



