PTUUIXOPECTEN EXTMIUS. 59 



Ptebinopecten eximius, de KonineJc, sp., 1885. Plate X, figs. 4, 5; Plato XI, 



figs. 13, 14. 



Aviculopecten eximius, de KonineJc, 1885. Ann. Mus. Roy. d'Hist. Nat. Belg., 



torn, xi, p. 211, pi. xxxvii, figs. 1, 2, 4, 5. 



Specific Characters. — Shell of medium size, suborbicular, the right valve 

 flattened, the left moderately gibbose. The margin of the valve orbicular, curved, 

 without a break from the base of each ear. The hinge-line straight, long, produced 

 posteriorly along the upper border of the posterior ear, and pointed. The anterior 

 ear the shorter, triangular, marked off from the valve by a well-marked groove in 

 the left, and by a slit in the right valve. The posterior ear not marked off from 

 the rest of the valve, but represented by a compression and extension of the shell, 

 so that in the upper part of the posterior border its contour is falcate. 



Interior. — Unknown. 



Exterior. — The surface is covered with radiating ribs, between each pair of 

 which a second rib soon rises and gradually increases in strength ; halfway across 

 the valve a third set of ribs arises, one between each primary and secondary rib. 

 The ribs are often in pairs, a large and a small one close together, and then an 

 interval. They are studded with close, rounded nodules, less apparent over the 

 posterior false ear. Concentric lines of growth are also seen passing across 

 the ribs. 



I> imc unions. — De Koninck's type specimen measures — 



Antero-posteriorly . . . .85 mm. 



Dorso-ventrally . . . .75 mm. 



Length of hinge-line . . . .75 mm. 



Localities. — England : the Carboniferous Limestone of Derbyshire. Ireland : 

 Lower Limestone Shales of Clonakilty, and Carboniferous Limestone of Little 

 Island, co. Cork; St. Doulagh's, co. Dublin. 



Observations. — This species is distinguished from P. granosns solely by its 

 ornament. Possessing the same general shape and size, the ornament is much 

 more regular, the ribs closer and more numerous, and the nodules on the ribs 

 rounder and closer than in the latter species. De Koninck describes the three 

 classes of radiating ribs, but he does not state that the secondary ribs may very 

 soon assume the same size as the primary ribs, so that at the margin of the valve 

 they are not to be distinguished. 1 have only seen one British example — a 

 specimen of a portion of the left valve from Derbyshire, in the Woodwaidian 

 Museum, Cambridge (PI. X, fig. 4). 



I am not persuaded of the value of P. eximius as a species, and think it very 



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