124 CARB0N1C0LA, ANTHRACOMYA, AND NAIADITES. 



The exterior is marked with exceedingly close, fine, microscopic, thread-like 

 stria?, 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. — PI. XVI, fig. 19, measures — 



Antero-posteriorly . . . .10 mm. 



Dorso-ventrally . . . .9 mm. 



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 Ixvis 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 Naladites Isevis. Judging from Dr. Dawson's figure, the concentric 



striae 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 to a 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 striae 



are finer and closer. From a study of young and adult forms of this genus the 



growth 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 Isevis 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 J. Isevis. Should this be the case, the name rugulosus, dating 



I s ;!'.), 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- 380 



Air. Etheridge, jun., also queries whether Hibbert's Unio nuciformis may not 



