162 CARBONIFEROUS LAMELLIBRANCHIATA. 



and produced forwards in front of the umbones. The umbones are small, convex, 

 incurved, non-contiguous, situated at the junction of the anterior and second 

 quarter of the hinge-line, and hardly raised above it. They are separated by a 

 narrow elongate ligamental area. The valves are evenly and regularly curved in 

 the greater part of their extent, but above and posterior to a line passing from 

 the umbo towards the posterior-inferior angle are rapidly compressed into the 

 posterior-superior angle, so that the dorsal slope becomes slightly concave. There 

 is a slight flattening of the valve to be observed in some specimens in the neigh- 

 bourhood of the byssal sinus. 



Interior. — The anterior adductor muscle-scar is situated some little way from 

 the anterior-superior angle in the anterior part of the umbonal hollow ; it is 

 shallow and round. The posterior adductor scar is in the hollow of the dorsal 

 slope, immediately below the end of the posterior hinge-teeth. The pallial line is 

 entire and remote from the margin. The hinge-plate is expanded before and behind, 

 and consists of a few small oblique teeth in front, inclined from above downwards 

 and backwards, and several (five to six) long narrow lateral teeth behind, all 

 terminating some distance from the posterior border, and the upper teeth being 

 longer than those immediately below. 



Exterior. — The surface is adorned with very fine concentric lines of growth, 

 with here and there a deeper one, decussated by a series of close, regular, radiating 

 lines, more strongly marked on the dorsal slope. Occasionally the radiating strias 

 appear to be absent. 



Dimensions. — PI. XI, fig. 25, measures — 



Antero-posteriorly . . . .17 mm. 



Dorso-ventrally . . . .12 mm. 



From side to side . . . .8 mm. 



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

 Yorkshire ; Narrowdale, Staffordshire. 



Observations. — This is one of the species described as Area by de Koninck in 

 the supplement to his first great work {op. supra cit.), and redescribed in 1885 as 

 Parallelodon. 



The Woodwardian Museum, Cambridge, possesses a series of four specimens 

 from the Carboniferous Limestone of Settle collected by the late Mr. Burrow, three 

 of which I figure, PI. XI, figs. 23, 25, 26. The last two specimens are almost 

 perfect examples, possessing both valves well preserved. The shells shown, PI. XI, 

 fig. 24, and PI. XII, fig. 14, obtained from Narrowdale by the late Mr. Carrington, 

 are in the Collection of the Geological Survey at Jermyn Street, and are chiefly 

 casts ; they fortunately exhibit the details of the muscle insertions and the hinge. 

 The shell, where the test is preserved, has not the radiating strias which usually 

 adorn the surface in this species. De Koninck noticed the fact, for of some of his 



