AVICULOPECTEN INTERSTITIAL JS. 95 



slightly convex. The hinge-line straight and elongate. The nmbones swollen 

 and pointed, snbcentral. The anterior ear in the left valve triangular, its 

 anterior superior angle produced, marked off by an angular groove from the 

 valve ; the posterior ear falcate. 



Interior. — The surface is smooth, showing radiating grooves corresponding 

 with the strong external ridges. Adductor scar shallow, round, and placed high 

 up in the valve, just posterior to the middle line. 



Exterior. — The surface of the left valve is marked with about six strong, 

 rugged, radiating ridges, between each pair of which are three or more finer 

 radiating lines, which become more apparent as the lower border of the valve is 

 approached. The centre ridge may reach the umbo. There are obscure concentric 

 folds. The anterior ear has a few obscure radiating folds, but the concentric 

 strise are well marked. The posterior ear is marked by concentric lines of 

 growth parallel with the contour of the ear. The right valve is ornamented with 

 numerous close, equal, rounded, radiating ribs, which bifurcate as they pass across 

 the valve. 



Dimensions. — PL XIV, fig. 16, measures — 



Antero-posteriorly . . . .35 mm. 



Dorso-ventrally . . . .32 mm. 



From valve to valve . . . .15 mm. 



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

 Wetton, Staffordshire ; Hill Bolton and Settle, Yorkshire ; Lowick, Northumber- 

 land ; Underset Limestone, Farcote Gill and Goodham Gill, Swarth Fell, 

 Westmoreland ; Poolvash, Isle of Man. . Scotland : the Upper Limestone series 

 of Garngad Road, Glasgow, and Bowertrapping, Shield ; New Cumnock, 

 Ayrshire ; Lennox Burn Quarry, Haddingtonshire ; shore east of Dalskelly Craig, 

 Boglehill, Longniddry; Smallburn, Kilsyth. Ireland: the Carboniferous Limestone 

 of Manor Hamilton, co. Leitrim ; Cornfield, co. Clare ; Clogherbrian, co. Kerry. 



Observations. — A. inter stitialis has been established on the peculiar characters 

 of the left valve. Phillips only describes this valve, and M'Coy, speaking of 

 A. segregatus, simply mentions that the right valve is " slightly convex." I have 

 been fortunate enough to obtain one specimen with the valves in contact, and although 

 much weathered, the contour and markings of the right valve are preserved. It 

 occurred in the upper beds of Carboniferous Limestone at Castleton, Derby- 

 shire (PI. XIV, figs. 16, 17). M'Coy's original figure appears as a right valve, but 

 this is an error due to the fact that no allowance was made for I lie reversal of the 

 figure when drawn on the stone. I quite agree with M'Coy that /'. Kolccharofi, 

 de Verneuil, is identical with his species, and should therefore be referred to 

 A. inter stitialis. I also think Pecten Bouei of that author is a younger specimen 

 of the same shell. De Koninck has placed A. seg regal us, M'Coy, as a synonym of 



