132 CARBONIFEROUS LAMELLIBRANCHIATA. 



form the highest point of the shell. There is no lunule, escutcheon, nor ridge; 

 but an obsolete, broad, oblique constriction at the junction of the anterior and 

 middle thirds of the valve. The greatest dorso-ventral depth is at the umbones, 

 and the greatest convexity at about the centre of the valve. 



Exterior. — The surface is almost smooth in the neighbourhood of the umbones ; 

 but elsewhere there are irregular concentric lines of growth, which become much 

 more apparent and frequent near the inferior margin. 



Dimensions. — The type specimen, fig. 28, PI. XIV, measures — 



Antero-posteriorly . . . .23 mm. 



Dorso-ventrally . . . .15 mm. 



Elevation of valve . . . .6 mm. 



Localities. — England: Bolland ? (Phillips); the Redesdale Ironstone, 

 Northumberland; in slate over the Grin Mine Coal, North Staffordshire. Scotland : 

 Craio-eno-len, Lower Limestone series. 



Observations. — The type-specimen of this species is in the Gilbertson Collection, 

 British Museum (Natural History), and I am permitted to refigure it (PI. XIV, 

 figs. 28, 28 a) by the kindness of the authorities. It is not quite complete at the 

 posterior end, and the left valve is unfortunately much distorted by crushing. In 

 the general collection is another fine specimen from Craigenglen, near Glasgow. 



M'Coy, re-describing this species, says (op. cit., p. 70), " This is a large 

 depressed species, chiefly remarkable for the obtuse diagonal ridge, which runs 

 from the beak to the front [sic] posterior angle." M'Coy substituted the name 

 V. Phillipsii for this species, he stating that a shell from some newer strata had 

 already received the same name. It is evident, from the stress laid upon the 

 obtuse diagonal ridge, that M'Coy was mistaking a totally different shell for 

 Phillips's N. undulata, the type of which has certainly no obtuse diagonal ridge. 



I doubt very much if the locality " Bolland," as given by Phillips, is correct ; 

 the shells here figured from the Gilbertson Collection were certainly never 

 obtained from a limestone matrix, but have all the appearance of the shells from 

 the Redesdale Ironstone-shales, in which beds I have myself obtained the species, 

 though its occurrence there is very rare. Nucula undulata is not likely to be 

 mistaken for any other Carboniferous species of the genus. It is much more 

 transversely ovate, more acutely pointed posteriorly, and larger than any of the 

 others. It appears that this species has nearly as great a vertical range as 

 N. gibbosa. 



