304 CARBONIFEROUS LAMELLIBRANCHIATA. 



de Koninck says " Cette espece a de grands rapports avec YEdmondia rudis, 

 F. M'Coy, qui s'en distingue par la situation anterieure de ses crochets et la forme 

 beaucoup plus arrondie des extremites de son bord cardinal." A large series of 

 specimens of E. rudis show that the actual position and degree of elevation of 

 the umbone is variable and depends largely on the stage of growth. In old shells 

 the umbones are less anterior and more elevated than when young. M'Coy's type 

 specimen is one which had only attained to medium growth, while the type of de 

 Koninck's E. rugata is much more fully grown. 



The Scotch examples are nearly all in the condition of internal casts. These 

 show the elongate narrow groove in the umbonal cavity which lodged the internal 

 ossicle, and other details of the interior. The internal surface was often as 

 rugose as the external, owing to the extreme thinness of the shell ; but the fine, 

 sharp lines of growth which are placed all over the rugged external surface are, of 

 course, wanting. 



The wide horizontal and vertical, distribution of this species is to be noted. 

 While few of the Lainellibranchs which occur in Carboniferous Limestone series 

 of Scotland are found in the Carboniferous Limestone of Yorkshire and Derby- 

 shire, E. rudis seems to have flourished equally well during the different conditions 

 under which each deposit was laid down, for it attains a full degree of develop- 

 ment in each locality, and only becomes dwarfed when it is found at a much 

 higher horizon, the Coal-measures. 



Edmondia compressa, M'Coy, 1844. Plate XXXIII, figs. 5—9. 



Edxiondia ? compeessa, M'Coy, 1841. Synopsis Carb. Foss. Ireland, p. 52, pi. xiii, 



fig. 10. 

 Caediomoepha compeessa, d'Orbigny, 1850. Prodrome de Paleontol., p. 133. 

 Edmondia compeessa, Morris, 1854. Cat. Brit. Foss., 2nd edit., p. 202. 

 — M'Coy, 1S55. Brit. Pal. Foss., p. 500. 



Bigsly, 1878. Thesaurus Devouico-Carboniferus, p. 307. 

 Etheridge, 1SS8. Brit. Foss., pt. 1, Palaeozoic, p. 283. 

 Compare — ? pe.elonga, de Koninck, 1S85. Ann. Mus. d'Hist. Nat. Belgique, p. 50, 



pi. vii, fig. 21. 



Specific Characters. — Shell transversely ovate, very inequilateral, compressed, 

 slightly oblique. The anterior end is very short, but deep, and only slightly con- 

 vex. The margin which forms a right angle with the hinge-line, descends at first 

 almost in a straight line, and then becomes very convex, sweeping round into the 

 inferior margin without a break. The inferior border is produced and convex, 

 especially behind, where it rises to join the posterior end, which is bluntly rounded. 

 The hinge-line is long and gently arched. 



