AVICULOPECTEN INEQTJALIS. 99 



The lower border rounded, its curvature being continued up to the base of the 

 posterior ear. Hinge-line short and straight. Umbones triangular, pointed, 

 central, incurved. Anterior ear much compressed, sharply defined from the 

 valve by a curved groove, the right ear slit for the byssus. Posterior ears small, 

 triangular, depressed, and flattened, also separated from the valve by a deep groove. 



Interior. — Unknown. 



Exterior. — The left valve is ornamented with from twelve to twenty thick, 

 radiating, nodular, distant ribs, which project as spines below the lower margin of 

 the shell in front ; between each pair of ribs is a thin, irregular, moniliform, linear 

 rib. The spaces between the ribs are smooth, but the whole surface of the valve 

 is irregularly nodular. The anterior ear has four or five radiating ridges, the 

 posterior also several. The latter has several spines projecting obliquely backwards 

 and upwards along its upper border. The right valve lias numerous close, rounded, 

 radiating ribs, on which are nodular swellings, caused by concentric undulations 

 crossing the ribs. The right anterior ear has about six distant, nodulose, radiating 

 ribs, the intervals between them showing close concentric lines. 



Dimensions.- — PI. XIV, fig. 3, the type, measures — 



Antero-posteriorly . . . .35 mm. 



Dorso-ventrally . . . .40 mm. 



Localities. — England: the Carboniferous Limestone of Castleton, Derbyshire; 

 Poolvash, Isle of Man. Ireland: the Carboniferous Limestone of Little Island 

 and Black Rock, co. Cork ; Tankardstown and Ardshanbally, co. Limerick. 



Obser rations. — The type of M'Coy's Pecten Murchisoni is preserved in the 

 Science and Art Museum, Dublin. It is a fine specimen of a left valve (PI. XIV, 

 fig. 3). The marking on the left valve is identical with that of Pterinopecten 

 Dumontianus, de Kon., sp., but the latter shell is at once distinguished by its long 

 hinge-line, the long, gradually compressed posterior ear, and the numerous ribs on 

 both ears. The nodulose character of the ribs is not very marked in the type. I 

 have noticed much variation in a suite of specimens from Castleton and Poolvash. 



An examination of the type of Pecten ovatus, M'Coy, preserved in the Museum of 

 Science and Art, Dublin, in the Griffith Collection, has led me to the conclusion 

 that the shell is the young of A. Murchisoni. The contour is the same, and so is 

 the general character of the ornament. M'Coy figures the secondary thin rib 

 between a pair of large ones in an enlarged view. The shell is so small that only 

 one secondary rib is figured. 



Aviculopecten inequalis, sp. nov. Plate XIV, figs. 14, 15. 



Specific Characters. — Shell of medium size, inequivalve, the right being much 

 smaller than the left valve, and somewhat flatter, slightly oblique. Antero-posterior 



14 



