330 ME. HERBERT BOLTON ON [Aug. I9II, 



examination ; and I am of opinion that the form ought to be 

 regarded as a variety of Anihracomya williamsoni, and not as a true 

 species. 



Horizons and localities. — Lower Coal-Measures ; 

 Speedwell Colliery, Kingswood, Bristol ; roof-shales over the Rock 

 or Badger Vein in the Second or Farrington Series, Radstock district. 



Upper Coal-Measures : Spoil-heap of Tyning Batch, Radstock. 



Anthracomya minima (Ludwig). (PI. XXYII, figs. 10-13.) 



This species occurs at Radstock ; at Speedwell Colliery, Kings- 

 wood ; and at three of the four faunal horizons lying between the 

 Ashton seams and the Bedminster Great Yein, at South Liberty 

 Colliery. The shells vary in length from 4 to 8 millimetres, and in 

 one instance the valves were found united. Specimens found in the 

 Second Series of the Upper Coal-Measures at Writhlington Colliery, 

 Radstock, are extremely well preserved in an earthy ironstone, and 

 show admirably the short hinge-line, the gentle curvature of the 

 valves, and the broad ovate outline of the shell. Wrinkling of the 

 jteriostracum is present, but of a finer character than in A. phillijpsi, 

 and it never masks the concentric lines of growth. Upon the 

 Upper Coal-Measure spoil-heap at Tyning Batch several good 

 examples were found in a dark shale, very similar in appearance 

 and character to a dark Upper Coal-Measure shale found at Foxcote 

 Colliery. The largest specimen obtained had an antero-posterior 

 length of 13 millimetres and a depth of 7. Good examples of 

 the species were obtained from a massive black shale in the 

 Upper Coal-Measures at Ludlows Colliery, where they occurred 

 in association with A. lanceolata. Two of the attached valves 

 show the whole length of the hinge-line and the umbones in close 

 apposition : a lunule also appears to be present. Quite a nume- 

 rous series was obtained from dark, compact, Upper Coal-Measure 

 shales at Foxcote Colliery, and all the specimens were in excellent 

 condition. 



Adult individuals of the species form in outline a fairly regular 

 oval, disposed somewhat obliquely to the hinge-line. The regu- 

 larity of this oval is but slightly broken dorsally, by the hinder 

 border of the hinge-line. The latter is straight, extending over 

 half the length of the shell in very young forms, while in older 

 examples it may occupy but a third of the length. The umbones 

 are small, gently rounded, moderately tumid, and lie opposite the 

 junction of the anterior and middle thirds of the hinge-line. 



The posterior ventral border is short, well rounded, and forms 

 the lowest part of the shell. From the posterior ventral border the 

 posterior margin passes forwards and upwards, sometimes almost 

 straight, sometimes with a slight sulcus, until it meets the hinder 

 end of the hinge-line in a definite and very obtuse angle. In young 

 forms, the umbones seem almost median to the hinge-line in 

 position. The surface of the valves is marked by a great number 

 of concentric lines of growth, much more closely packed than in 

 Anthracomya phillipsi. They are also clearly independent of 



