AVICULOPECTEN PLANOCLATHRATUS. 91 



Interior. — Unknown . 



Exterior. — The surface is ornamented with 15 to 20 radiating ribs, not 

 always of equal strength, crossed at regular intervals by about 13 concentric 

 ribs, dividing the surface into distinct quadrilateral spaces. The ears marked by 

 imbricating, concentric lines. In the right valve the concentric ribs are less well 

 marked and are often obsolete. 



Dimensions. — PI. XVI, fig. 1, measures — 



Antero-posteriorly . . . .19 mm. 



Dorso-ventrally . . . .17 mm. 



Convexity of left valve . . . .2 mm. 



Localities. — England : the Carboniferous Limestone of Settle, Yorkshire ; 

 Castleton, Derbyshire ; Lowick, Northumberland. Ireland : the Carboniferous 

 Limestone of Little Island, co. Cork ; Killymeal, Dungannon, co. Tyrone. 



Observations. — De Koninck's species, Pecten nobilis, has the right of priority 

 over M'Coy's P. cognatus, which de Koninck recognised as a synonym of his shell. 

 At first glance this species might be mistaken for Pterinopecten tessellatus, Phill., 

 sp., but the latter is much broader, has no distinct posterior ear, and obviously 

 fewer concentric lines and ribs. The two species are therefore generically distinct. 



Specimens of A. nobilis vary considerably as to the number and strength of the 

 radiating and concentric ribs. I am not at all sure that Aviculopecten Forbesii of 

 de Koninck belongs to the species under discussion. 



A. cancellatulus, M'Coy, is the young form of de Koninck's species. The 

 Woodwardian Museum, Cambridge, possesses a fine series from Settle, showing 

 both right and left valves in various stages of growth. The confusion of the 

 various species now referred to A. nobilis has doubtless arisen from the varying 

 characters of the ornament due to the strength, number, and degree of development 

 of the radiating ribs and concentric ridges. M'Coy states that A. cancellatulus is 

 distinguished from all other species " by the very large spherical or conoidal nodules 

 at the intersection of the concentric and radiating ridges." I have noted this 

 character in A. nobilis, especially in the younger part of the valves. I think that 

 P. sequalis, M'Coy, the type of which is a very poor specimen, is only the young of 

 A. nobilis. The types of both P. sequalis, M'Coy, and P. cancellatulus, M'Coy, are 

 from the same locality and horizon, Killymeal, Dungannon. 



Aviculopecten planoclathrattjs, M'Coy, sp., 1844 Plate XV, figs. 8 — 12. 



Pecten planoclathratus, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 98, 



pi. xvi, fig. 2. 

 Cf. Aviculopecten tenuilineatus, de Koninck, 1885. Ami. Mus. Roy. d' I list. Nat. 



Belg., torn, xi, p. 224, pi. xxxv, fig. 31. 

 13 



