92 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



Specific Characters. — Shell obliquely subquadrate, compressed, and expanded. 

 Umbones obtuse, much nearer the anterior end than in A. Adamsii. The anterior 

 end is almost obsolete and very obtuse, on which account the oblique sinus is 

 absent. The diagonal swelling is broad, and not well differentiated from the rest 

 of the shell. The anterior, inferior, and posterior borders form part of one 

 general curve, which meets the inferior border posteriorly at an obtuse angle. 

 There is no relic of a byssal notch. In other points the shell agrees with the 

 general description of A. Adamsii. 



Dimensions (PL XIII, fig. 9) : 



Antero-posteriorly . . . .53 mm. 



Dorso-ventrally . . . .42 mm. 



Laterally . . . . .8 mm. 



Localities. — The same beds in the North Staffordshire Coal-field in which 

 A. Adamsii occurs. 



Observations. — From the description and figure in the ' Geology of Coalbrook- 

 dale ' of Unio dolabratus, at first sight it might be considered that the variety 

 under description was referred to, but owing to an imperfect example a partly 

 erroneous diagnosis was given. Salter, in his observations on A. Adamsii, says 

 that A. dolabrata, Sow., may be distinguished by having an almost obsolete 

 anterior end, but he did not notice the fact, well shown in the drawing, that the 

 anterior end was incomplete through fracture. 



A. Adamsii, var. expansa, also differs from A. dolabrata in possessing a much 

 less convex but more expanded form. Fig. 11, PI. XII, is a form of intermediate 

 character between A. Adamsii and the var. expansa. It possesses the general 

 shape of the variety, but the position and shape of the umbones is very remarkable ; 

 they are more posteriorly situated than in any other specimen of A. Adamsii that 

 I have met with, and are at the same time more defined, more tumid, and raised 

 to a greater height above the hinge-line. This shell is not crushed in any way, 

 though it was split in two in extracting it from its ironstone matrix. A large 

 crushed specimen of the variety expansa which I figured in my former paper came 

 from the shale above the Burn wood ironstone at Pitts Hill, and measured 65 mm. in 

 length, and 55 mm. in the dorso-ventral diameter. It may be noted that at Pitts 

 Hill, Golden Hill, and Kidsgrove the top portion of the Burnwood ironstone differs 

 lil liologically and palasontologically from the lower part. The top part consists of 

 thin lamina3 of brown and lighter coloured ironstone, passing up into a black shale, 

 and containing Anthracomya Adamsii and its variety expansa, and a lower more 

 compact mass without marked lines of deposition, which contains Anthracomya 

 pulchra and a small Naiadites, the two forms of Anthracomya never occurring 

 together. On the other hand, at Great Fenton I cannot make out this sharp 

 division, and have never seen A. pulchra in the bed at this place. 



