PSEUDAMUSIUM ANISOTUM. 105 



Interior. — The adductor muscle-scar large and normal in position. The lower 

 part of the left valve obscurely ribbed internally. 



Exterior. — The surface of both valves is quite smooth, with an occasional con- 

 centric sulcus, pointing to interference with growth. Several specimens show 

 narrow, radiating, and concentric colour-bands. Both anterior ears are crossed by 

 several moniliform radiating- ribs. 



Dimensions. — PL XXI, fig. 13, the type, measures 



Antero-posteriorly . . . . 14 mm. 



Dorso-ventrally . . . . 16 mm. 



Localities. — England : the Carboniferous Limestone of Thorpe Cloud, Park 

 Hill, and Castleton, Derbyshire ; Narrowdale, Staffordshire ; Hill Bolton and 

 Settle, Yorkshire ; five feet above Underset Limestone, Goodham Gill, Swarth 

 Fell, Westmoreland ; White Limestone, Poolvash, Isle of Man. Scotland : shore 

 east of Kinghorn ; shore west of Kinning Point, Charlestown ; Cockmuir Quarry, 

 Hillhead Bridge, Denny. Ireland : Carboniferous Limestone of Little Island and 

 Ballydaniel, co. Cork ; Doohylebeg, co. Limerick. 



Observations. — The type-specimen (PI. XXI, fig. 1 3) is preserved in the Gilbertson 

 Collection, Natural History Museum, South Kensington. It is a right valve and 

 not very perfect. The species is easily recognised by the smooth surface of both 

 valves, and it is very common in the shell beds on Thorpe Cloud, Dovedale, but it 

 is difficult to obtain whole from the matrix. The valves vary a good deal in breadth, 

 and there is a tendency for the shells to become less ovate and more orbicular with 

 age, and I imagine that several of de Koninck's species represent different states 

 of P. anisotum. De Koninck referred the species to Aviculopecten in the text, but 

 with a (?) in the explanation of the plates ; I am at a loss to understand why he 

 retained the species in the genus Aviculopecten. P. anisotum is more oblique, less 

 tumid, and broader than P. ellipticum. 



P. sibericum, de Verneuil, is, I think, probably the same shell as Phillips's 

 P. anisotum. The drawing of the Russian shell represents a right valve, showing an 

 anterior ear, which has a reticulate character; this and the smooth, slightly 

 sulcated, broad ovate valve, seem to me to leave little doubt as to the identity of 

 the shell. 



I consider that Pecten consimilis, M'Coy, is merely the young of Phillips's shell. 

 M'Coy called attention to the striatum of the anterior ears, which is a characteristic 

 feature. I think that P. variabilis, M'Coy, represents a rather medium-sized 

 specimen of Phillips's species, but the type-specimens are so obscure that their true 

 nature is quite uncertain. Lima obliqua, M'Coy, also seems to me to be a fragment 

 of P. anisotum. 



