138 CARBONIFEROUS LAMELLIBRANCHIATA. 



Interior. — The hinge-plate appears to be normal. 



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

 centric lines of growth are to be seen. 



Dimensions. — PL XXIII, fig. 12, a right valve, measures — 



Antero-posteriorly . . . .35 mm. 



Dorso-ventrally . . . .14 mm. 



Locality. — A bed of shale above the Underset Limestone, Nine Standard Rigg, 

 Westmoreland. 



Observations. — P. angustus is distinguished from all other species of the genus 

 by its smooth surface and long antero-posterior diameter. It is not rare at the 

 locality mentioned above, where it is associated with the fauna enumerated on 

 p. 358, Vol. I, supra. P. obtusus has an almost smooth surface, but is not so 

 narrow and elongated as P. anqustus. 



Fatjai.lelodon elongatiis, sp. nov. Plate XXIV, fig. 16. 



Specific, Characters. — Shell above medium size, transversely sub-trapezoidal, 

 gibbose. The anterior end rounded. The inferior margin almost straight, parallel 

 to the hinge-line, the posterior margin truncate, slightly oblique, forming well- 

 marked angles with hinge-line above and the inferior border below. The hinge- 

 line long and straight. The umbones are elongate, gibbose, incurved, and placed 

 in the anterior fourth of the valve. Proceeding obliquely downwards and back- 

 wards from the umbo to the postero-inferior angle is a well-marked obtuse ridge, 

 above which the shell is so much compressed as to form a hollow dorsal slope. 

 The anterior part of the valve is regularly convex from above downwards. 



Interior. — The interior is smooth and shows no indication of radiating ribs. 



Exterior. — Unknown. 



Dimensions. — PI. XXIV, fig. 1G, a left valve, measures — 



Antero-posteriorly (estimated) . . .70 mm. 



Dorso-ventrally . . . .25 mm. 



Locality. — From the Knipe Scar Limestone, Shap, Westmoreland. 



Observations. — The elongate form and comparatively narrow dorso-ventral 

 diameter, the very strong oblique ridge and hollow dorsal slope, distinguish this 

 species from others of the genus, especially from P. obtusus, which is perhaps that 

 one which has most resemblance to it. P. elongatus occurs very low down in the 

 Carboniferous Series, the Knipe Scar Limestone being the second limestone from 

 the base of the Carboniferous in Westmoreland. 



