234 CARBONIFEROUS LAMELLIBRANCHIATA. 



oval, situated at the antero-superior angle of the valve, bounded below and 

 behind by an almost obsolete curved ridge ; the posterior is larger, oval, and mar- 

 ginal, situated at the postero-superior angle. There are some obscure radiating, 

 almost obsolete ridges in the hollow of the oblique ridge, represented by grooves 

 in casts. The pallial line is entire, and close to the margin. 



The hinge has not yet been exposed in British examples, but de Koninck 

 describes the right valve as having a single tooth anterior to the umbo, and the 

 left valve two teeth, separated by a deep socket, an example of which I figure 

 PL XVII, fig. 15 a. 



Exterior. — For the greater part the surface is almost smooth, but towards the 

 inferior margin irregular concentric grooves with subimbricating lines and ridges 

 appear, which follow the contour of the valves. 



Dimensions. — Fig. 7, PI. XVIII, from Orchard, near Glasgow, a testiferous 

 example, measures — 



Antero-posteriorly . . . .32 mm. 



Dorso-ventrally . . . .27 mm. 



Elevation of valve . . . .8 mm. 



Localities. — England : the Limestone of Lowick, Northumberland. Scotland : 

 Upper Limestone series ; Orchard, in the Thornliebank district, and East Kilbride, 

 near Glasgow. 



Observations. — The Woodwardian Museum, Cambridge, possesses a fine series 

 of this species from the neighbourhood of Lowick, but the exact limestone of the 

 many in this locality from which they were obtained is uncertain. These speci- 

 mens are all in the form of casts, but have the internal characters of the 

 shell beautifully preserved. Fortunately a testiferous example, from Orchard, 

 in the collection of Mr. J. Neilson, has enabled me to describe the exterior of the 

 valve. I have been favoured with the loan of the shell figured by the Rev. David 

 Ure as a " cockle," from Black Craig, East Kilbride, now in the possession of the 

 Royal Society of Edinburgh, and which is here figured, PI. XIX, fig. 1. This shell 

 was referred by John Gray to Axinus carbonarius, op. cit., but it is quite distinct 

 from the shells figured by Portlock under the same specific title. 



Three species — P. impressus, P. Halli, and P. uncinatus — described by 

 de Koninck so closely resemble each other, that I think it very probable that 

 they only represent one species. Of P. Halli de Koninck states, " Cette espece 

 est tres voisine du Protoschizodus impressus, dont elle ne differe que par une forme 

 un peu moms longue et moins anguleuse ; les dents cardinales de la valve gauche 

 sont aussi un peu moins saillautes que celles de cette meme valve de l'espece qui 

 vient d'etre citee" (P. impressus). Of the latter species he states, "Elle a aussi 

 des rapports avec le Protoschizodus uncinatus, que s'en eloigne par une forme plus 

 allongee et par la corbure plus reguliere et plus intense de son bord ventral." I 



