330 CARBONIFEROUS LAMELLIBRANCHIATA. 



appears to be obsolete. The binge-plate consists of a thickened ridge leaving a 

 groove in casts. Pallial line entire. The internal surface of the shell is deeply 

 marked with concentric grooves and ridges corresponding to those on the 

 surface. 



Exterior. — The surface is ornamented with regular, broad, angular ridges, twelve 

 to fifteen in number, separated by concentric grooves which enlarge rapidly as 

 they pass across the shell, and become broader as they approach the lower 

 margin, the ridges and grooves being themselves covered with fine concentric 

 lines of growth, which are variable as to size and distance apart. Shell very 

 thin. 



Dimensions. — PI. XXXVI, fig. 24, measures — 



Antero-posteriorly . . . .32 mm. 



Dorso-ventrally . . . .25 mm. 



Elevation of valve . . . .10 mm. 



Localities. — England: the Carboniferous Limestone of Settle, Yorkshire; 

 Thorpe Cloud, Castleton and Park Hill, Derbyshire; Narrowdale, Staffordshire ; 

 The Coomb and Lowick, Northumberland ; the Cayton Gill beds near Harrogate 

 in the Millstone-grit series. Scotland : the Lower Limestone of Auchenskeith 

 and Dockra. Ireland : Carnteel, Tyrone. 



Observations. — This species was described as E. scalaris by M'Coy in his 

 second great work (op. supra eit.), but it differs markedly from the original type, 

 as pointed out by De Koninck. The question of the affinities and differences of 

 the two species has been discussed under my observations on E. scalaris, and 

 need not be repeated here. 



De Koninck has described a shell, under the name E. scalar if ormis, somewhat 

 the shape of E. MacCoyii, but it is rather more transverse. It is quite possible that 

 the two forms may be nothing more than varieties of the same species, but at 

 present I have not access to enough material to venture on any definite statement 

 with regard to them. I may say that the only difference between the shells is that 

 of relative measurements, De Koninck's E. scalariformis having its anfero-posterior 

 diameter 40 mm. and dorso-ventral 26 mm. ; in E. MacCoyii the diameters are as 

 32 : 25. 



There appears to have been a great amount of variation in the size, number, 

 and proximity of the concentric ribs. M'Coy's type shows only about ten, while 

 fig. 28, PL XXXVI, a specimen from Settle, shows about thirty ; but examples 

 with much fewer ribs occur in the same locality. 



In shape E. MacCoijii resembles E. rudis very closely, and the more regularly 

 marked examples of the latter species would appear to pass into the more closely 

 ribted forms of the former. The extremes are, however, so distinct that I have 

 not hesitated to retain both species. 



