150 CARBONIFEROUS LAMELLIBRANCHIATA. 



Interior. — Muscle-scars normal in position. Hinge-plate very indistinctly 

 known, but with two or three long, posterior lateral teeth. Internal surface 

 smooth, with obsolete radiating lines. 



Exterior. — The surface is ornamented with very fine concentric lines, here and 

 there one being so pronounced as to be subimbricating ; but the concentric 

 markings are over the posterior two-thirds of the valve quite secondary in appear- 

 ance to the radiating lines, which are fine, straight, regular, and under the micro- 

 scope punctate. In the anterior third of the shell the concentric lines are the most 

 prominent, and these are very irregular in the line of the byssal sulcus. Shell 

 thin. 



Dimensions. — PI. X, fig. 13, measures — 



Antero-posteriorly . . . .38 mm. 



Dorso-ventrally (at umbo) . . .23 mm. 



Elevation of valve . . . .8 mm. 



Localities. — The Carboniferous Limestone of Settle and Hill Bolton, Yorkshire ; 

 Castleton, Derbyshire. Scotland : Pot-metal Plantation, Bogie, Kirkcaldy. 



Observations. — This species has not been previously noted in the Carboniferous 

 beds of Great Britain, but has been confounded with P. tenuistria, which it 

 resembles somewhat. But it is easily distinguished by the broad posterior end, 

 and the presence of well-marked concentric lines of growth. I think that most 

 of the species referred by de Koninck to P. tenuistria should, for this reason, more 

 properly be placed under P. Walciodorensis. 



Remarking on P. tenuistria, de Koninck says (p. 162), " Quoique les ornements 

 dont la surface de cette espece est couverte soient un peu plus apparents que ceux 

 que M. F. M'Coy a fait representor par le dessin qu'il a publie de sa Cucullsea 

 tenuistria, je ne crois pas me tromper en identifiant avec elle l'espece que je viens 

 de decrire ; elle n'a d'ailleurs de l'analogie qu'avec le P. Walciodorensis, qui s'en 

 distingue par une hauteur relativement moins grande et par la profondeur plus 

 forte des stries qui servent a, produire les ornements garnissant sa surface." It is 

 evident, therefore, that the essential difference in the contour of the posterior end 

 was not grasped by de Koninck, probably because he had only specimens of 

 P. Walciodorensis for examination. 



P. Laeordairianus, which has some resemblance to the species under discussion, 

 is distinguished by the less transverse, more quadrate form, but narrowed posterior 

 end, large ligamental area, blunter gibbose umbones, and greater convexity of the 

 valves. 



The University of Cambridge possesses a fine series of P. Walciodorensis in 

 various states of growth, all obtained by the late Mr. Burrow from the Carboni- 

 ferous Limestone of Settle, and some of these I have been permitted to figure, 

 PI. X, figs. 11, 12. 



