18 CARBONIFEROUS LAMELLIBRANCHIATA. 



Lifrrior. — Aiiterioi' adductor muscle-scar small and round, single, remote from 

 the anterior extremity. Hinge-line straight, narrow, with oblique teeth in front and 

 grooved posteriorly. Pallial line remote from border, very slightly marked. 



Exterior. — The surface is almost smooth, but under the microscope fine concen- 

 tric lines and striae of growth are visible . 



Dimcii.siouft. — PI. Ill, fig. 10, from the Carboniferous Limestone of Hill Bolton, 

 measures — 



Antero-posteriorly . . . .40 mm. 



Dorso-ventrally (at posterior end) . . 20 mm. 



From side to side (estimated) . . .10 mm. 



Localities. — England : the Carboniferous Limestone of Settle and Hill Bolton, 

 Yorkshire, and Lowick, Northumberland. Scotland : Potmetal Plantation, Fife ; 

 Upper Limestone series, Thornliebank. Ireland : smooth non-calcareous shale, 

 Fermanagh. 



Observations. — Portlock's figured specimen of Pterinea Thompsoni, PI. IV, 

 fig. 12, and tAvo others from the same locality, are preserved in the museum of 

 the Geological Survey, Jermyn Street. They are young specimens and crushed, 

 but the characteristic contour is well preserved. The species seems to be 

 fairly widely distributed, and it is common at Hill or El Bolton, Yorkshire. 

 Leiojdfria Thompsorii is more regularly and transversely triangular than any other 

 species of the genus, and can in this way be easily recognised. De Koninck 

 has figured and referred to three species, shells which I believe to be nothing more 

 than different stages of growth of Portlock's shell. I have, therefore, regarded 

 these names as synonyms. It will be noted, too, on referring to de Koninck's 

 figures of L. trigoiiaJis and L. vwdiolaris, that the former is incomplete in front, 

 and the latter is imperfect behind, and that the contour of the lines of growth in 

 the younger portion of the shell does not warrant the restoration of the outline as 

 depicted. L. narinila, another of de Koninck's species, is said to differ from //. 

 viodiolaris simply in the surface markings, and it seems to me highly probable that 

 it is the right valve of the other species. L. emaciata seems to be only a 

 young example of the same species. The anterior end is, though narrow, well 

 marked, and separated from the body of the valve by a constriction. 



