AVICULOPECTEN INCRASSATUS. 89 



valve (PI. XVI, fig. 7), with the posterior ear broken off and the anterior ear only 

 half developed, but shows well the bold character of the radiating ribs. It will be 

 noticed, on examining the specimen, that the secondary ribs are few, and commence 

 low down on the valve. This I regard as an abnormality, due probably to 

 insufficient nourishment or want of carbonate of lime. It is owing to this fact, 

 probably, that de Koninck and M'Coy described this species under other names. 



Aviculopecten villanus, de Kon.,has the peculiar circular-shaped valve, the large 

 ears, and long hinge-line pointed at each end, and simply varies from Phillips's shell 

 in that the secondary ribs start regularly high up, not far from the beak, and 

 rapidly become equal in size to the primary ones. De Koninck figures the right 

 and left valves. He points out that the former has more numerous and finer 

 radiating ribs, and is flatter than the left valve. 



Aviculopecten incrassatus, M'Coy, sp., 1844. Plate XVI, figs. 12 — 15. 



Pecten incrassatus, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 94. pi. xvi, 



fig. 1. 

 Aviculopecten illegalis, M'Coy, 1855. Brit. Pal. Foss., p. 486. 



— megalotis, de KonincJc, 1885. Ann. Mus. Koy. d'Hist. Nat. Belg., 



torn, xi, p. 230, pi. xxxvi, fig. 25. 



Specific Characters. — Shell below medium size, inequivalve, suborbicularly 

 quadrate, the left valve regularly and moderately convex, the right valve less 

 so. The curvature of the margin from ear to ear oblate. The hinge-line straight 

 and prolonged specially backwards, rolled. The umbones subcentral, that of the 

 left valve moderately convex, pointed, very slightly raised; the right much less 

 pronounced, triangular, and pointed. The anterior ears depressed, separated 

 from the body of the valve by an oblique sulcus, more linear and acute in the 

 right valve. The anterior margin of this ear rounded. The posterior ears long, 

 pointed, falcate, formed by a gradual compression of the valve. 



In tenor. — Unknown. 



Exterior. — The surface of the valve is adorned with many thick radiating ribs, 

 between each pair of which a secondary rib arises, about halfway across the 

 valve or sooner. The ears are almost smooth, the posterior marked with faint 

 concentric lines and striae, but no radiating ribs. The right valve is very similar 

 to the left valve. 



Dimensions. — PI. XVI, fig. 15, the type, measures — 



Antero-posteriorly .... 33 nun. 

 Dorso-ventrally . . . . :]] mm . 



Localities.— England : the Carboniferous Limestone of Settle and Malham 



