124 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
The ewterior is marked with exceedingly close, fine, microscopic, thread-like 
striz, which are closely crowded in front, but become parallel to the margin, and 
terminate in the upper border. Obscure radiating lines are sometimes present. 
Dimensions.—P|. XVI, fig. 19, measures— 
Antero-posteriorly : : 5 | 10smm: 
Dorso-ventrally : . = | .0immr 
Localities.—Scotland: Limehouse Water, near Mid Calder; Straiton oil shales 
near Burdiehouse; Shale, Inchkeith Island; Quarry, east of Binn Hill, Burnt 
Island, Fife; Middleton Pit, Uphall, Edinburghshire, all in the Cement Stone 
group near the horizon of the Burdiehouse Limestone; Wardie Shales, Water of 
Leith, Kates Mill, near Edinburgh at a lower horizon. England: Trafalgar Pit, 
Cinderford, Forest of Dean. Roof of Cannel Coal, Wigan. Spirorbis Limestone, 
Ardwick. 
Observations.—Mr. Robert Etheridge, jun., who was the first to describe the 
shell now under consideration, drew attention to the close resemblance between it 
and Anthracomya levis of Dawson. He states that ‘ I was for some time quite at 
a loss how to distinguish between them; however, A. scotica is larger than the 
Nova-Scotian Naiadites levis. Judging from Dr. Dawson’s figure, the concentric 
strie are also more numerous, finer, and closer; the posterior end appears to be 
more obliquely truncate, and the beaks are more anterior.” These distinctions seem 
to me to be due to a large extent toa difference in size, and hardly to be characters 
of specific value. I have had the opportunity of examining specimens of the Nova 
Scotian form, and those that I have seen are only half the size of the Scotch 
examples, and are, as Mr. Etheridge states, somewhat more oblique, and the striz 
are finer and closer. From a study of young and adult forms of this genus the 
erowth always appears to be more rapid towards the posterior end, and the 
tendency is for shells as they increase in size to become less oblique. It would 
be natural, too, to suppose that in the smaller examples the lines of growth would 
be finer and closer. I have, therefore, considered it best to retain Sir W. J. 
Dawson’s name of /evis for this shell, adding the name scotica as denoting a large 
variety. Mr. Salter (op. cit., p. 80) identifies Dawson’s shell with one found in the 
Upper Coal-measures of the neighbourhood of Manchester. This may perhaps be 
the form named Unio rugulosus of Phillips, the description of which seems to 
agree very well with A. levis. Should this be the case, the name rugulosus, dating 
1839, must supersede all others. The description of U. rugulosus is ‘* of obliquely 
expanded or semi-elliptical form. The hinge-line forming the diameter [of both 
valves]. Surface concentrically marked with broken undulations, often showing 
radiations on the posterior slopes. Shell exceedingly thin.” (‘Sil. Syst.,’ 
p. 88.) 
Mr. Etheridge, jun., also queries whether Hibbert’s Unio nuciformis may not 
