AVICITLOPECTEN TABULATUS. 67 



the left valve of the shell previously described by him as Pecten tabulates. M'Coy 

 included in his genus shells which are now separated under Pterinopecten, Hall, 

 and, as understood by him, it evidently included shells of widely different 

 characters. De Koninck followed M'Coy, and refused to accept Hall's genus 

 Pterinopecten, or HJuchondria, Meek; and Zittel, taking P. papyraceus as the type of 

 Aviculopecten, seems to have been of the same view. 



I think, however, there can be little or no doubt as to the wisdom of dividing 

 ovate shells with well-marked posterior ears from semicircular forms with 

 posterior ears undefined. In Aviculopecten the right valve is always flatter than 

 the left, and has a different marking. The tendency is for the radiating ribs to 

 be best marked in the left valve, and concentric lines in the right valve. In 

 Pterinopecteii the right valve is almost flat, but the ornament is essentially the 

 same as that of the left valve, though it may be less strongly marked. 



Aviculopecten seems to have been richer in species than any of the other 

 Pectiniform erenera found in Carboniferous rocks. 



Aviculopecten tabulatus, M'Coy, 1844. Plate XII, figs. 1 — 4. 



Pecten tabulatus, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 100, pi. xvi, 



fig. 12. 

 Aviculopecten planoradiatus, M'Coy, 1851. Ann. Mag. Nat. Hist,, ser. 2, 



vol. vii, p. 171. 

 — 1855. Brit. Pal. Foss., p. 489, pi. 3 e, 

 fig. 8. 

 ? B. Etheridge, jun., 1876. Geol. Mag., doc 2, 



vol. iii, p. 151. 



Specific Characters. — Shell of medium size, triangularly pyriform, left valve 

 gibbose, the right valve much less so. The ears large and depressed, the anterior 

 triangular, in the right valve separated from the body of the valve by a dee]) 

 groove, and rolled, in the left valve flattened and depressed. The posterior ear 

 large, flattened, much hollowed by an oblique groove at its junction with the body 

 of tht' valve, and having the upper border much produced, and its border 

 markedly falcate. The lower margin of the posterior ear descending and cutting 

 the posterior border a little below half the dorso-ventral diameter. The lower 

 margin of the valve rounded; the anterior margin below the ear almost straight, 

 directed obliquely forwards and downwards. The hinge-line straight and very 

 long. The umbones swollen, incurved, central in relation to the body of the valve ; 

 the left umbo the more swollen. 



Interior. — The internal surface is smooth, except near the lower margin, where 



10 



