NUCULA UNDULATA. 181 



gibbose form, and the approach to squareness in the anterior-inferior 

 angle. 



The distribution of N. gibbosa is of the greatest interest. The lowest occur- 

 rence of this species seems to be the Calciferous Sandstone series of Fifeshire, 

 where Mr. J. W. Kirkby l estimates that it ranges from 500 to 2300 feet below the 

 horizon of the Hurlet Limestone. The species then seems to range freely at 

 various horizons in the Carboniferous Limestone series of Scotland up to the 

 shales below the Linn Limestone and the shales over the Robringston Limestone, 

 of Lanarkshire, above which the species does not seem to recur. 



In England N. gibbosa is found in one bed at least, the Redesdale Ironstone, 

 in the Carboniferous Limestone Series of Northumberland ; but from this horizon 

 upwards it disappears, and is not found in the great " mass " of limestone of 

 Central England. After this tremendous break it reappears, sporadically, 

 however, in the shales below the third bed of Millstone-grit at Congleton Edge, 

 and in the shale above the Gin Mine Coal of the Middle Coal-measures, North 

 Staffordshire. I figure a specimen from the latter horizon, PI. XIV, fig. 13,. 

 from the cabinet of Mr. John Ward, F.G.S. 



Nucula undulata, Phillips, 1836. Plate XIV, figs. 28—31 ; Plate XV, fig. 33. 



Nucula undulata, Phillips, 1836. Geol. Yorks., part 2, p. 210, pi. v, fig. 16. 



— — Morris, 1843. Cat. Brit. Foss., p. 95. 



— Phillipsii, M'Coy, 1845. Carb. Foss. Ireland, p. 70. 

 Cardiomoepha tjndulata, d'Orbigny, 1850. Prodr. Paleontol., p. 132. 

 Nucula undulata, Brown, 1849. Illust. Foss. Conch., p. 189. 



— — de EycJcholt, 1851. Melang. paleont., p. 152, pi. xvii, fig. 7. 

 Ctenodonta Phillipsii, Bigsby, 1875. Thesaur. Devonico-Carboniferus, p. 303. 

 Nucula undulata, Etheridge, 1885. Brit. Foss., part 1, Palseont., p. 287. 



Specific Characters. — Shell transverse, ovoid, moderately but regularly convex. 

 The anterior end is somewhat compressed, and its border is regularly rounded, 

 but narrowed from above downwards. The inferior border is very convex 

 downwards, the central point of the convexity being slightly posterior to 

 a vertical line dropped from the umbo. The posterior end is somewhat 

 attenuated, and the border bluntly rounded. The superior border is angular, 

 the posterior limb being longer than the anterior, the two meeting at a very obtuse 

 angle below the umbones, which are small, pointed, contiguous, hardly marked off 

 from the surface of the shell, not raised above the hinge-line, and situated a 

 little posterior to the junction of the anterior and middle thirds of the shell. They 



1 ' Quart. Journ. Geol. Soc.,' vol. xxxvi, p. 589. 



