244 CARBONIFEROUS LAMELLIBRANCHIATA. 



angular ridge, above which the shell is compressed, forming a flattened dorsal 

 slope. The valve is gently convex, and much narrowed posteriorly from above 

 downwards. 



Interior. — The pallial line is entire. The hinge of the right valve consists of a 

 single fairly large tooth, situated in front of the umbones, inclined forwards. 

 The posterior part of the hinge-plate is somewhat flattened, but there is no sign 

 of a posterior lateral tooth. Muscle-scars are normal in position. Pallial line 

 simple. 



Exterior. — The surface is almost smooth, but here and there the microscope 

 reveals fine concentric lines of growth. Shell very thin. 



Dimensions. — Fig. 10, PI. XIX, measures — 



Antero-posteriorly . . . .13 mm. 



Dorso-ventrally . . . 9 mm. 



Elevation of valve . . . .4 mm. 



Localities. — Scotland : Encrinite-bed, Broom Hill, St. Andrews ; Calciferous 

 Sandstone series. Ireland : Carboniferous shales of Dromard, Draperstown, co. 

 Londonderry. 



Observations. — This little species was described and figured by M'Coy in 1844. 

 The Woodwardian Museum, Cambridge, possesses a slab from the same locality as 

 that where the type specimen was collected, which is full of specimens of this 

 shell. I have been able to isolate the hinge of the right valve, which I have 

 described above, and which is typical of the genus in its characters. The 

 Geological Survey of Scotland possesses a block from the Encrinite-bed, 

 St. Andrews, in which are two specimens, both casts. One, a right valve, shows 

 as a hollow the place which the cardinal tooth occupied ; the other, a left valve, 

 gives details of the muscle-scars and pallial line. 



This species is much more comparatively transverse than any other, and can 

 be easily distinguished by this character. 



The Anatina deltoidea, M'Coy, is, I think, without doubt, a synonym of this 

 species. The following is the description given of it : — " Subtrigonal, gibbous ; 

 anterior side large, rounded ; front margin very convex ; posterior side com- 

 pressed, broad, obliquely truncated, separated from the body of the shell by an 

 obtuse ridge," <fcc. The interior is shown in the original figure with an entire 

 pallial line; but it is stated that the "pallial impression (is) sinuous." An 

 attempt is made in the figure to make the pallial sinus in an impossible place, 

 i. e. posterior, and just below the posterior adductor muscle. This specimen is 

 refigured, PI. XIX, fig. 11. Although the name Anatina deltoidea is found on p. 51, 

 and that of Axinus nuculoides on p. 63, the former specific appellation cannot be 

 adopted on account of its previous use (though only as a synonym) for another 

 species of this genus. 



