AVICULOPECTEN KNOCKONNIENSIS. 85 



Specific Characters. — Shell small, longitudinally ovate, moderately convex; ears 

 comparatively large ; both valves with somewhat similar ornament, the right flat, 

 the left convex. The anterior margin of the left valve and the posterior margins 

 of both somewhat falcate. The anterior margin of the right valve deeply slit for 

 the byssns below the ear. The lower margin rounded. The hinge-line com- 

 paratively short, produced along each ear and pointed. The umbones small, 

 triangular, and pointed, almost central. The anterior ear of right valve large, 

 its margin rounded, separated from the rest of the valve by a hollow sulcus; that 

 of the left valve triangular and well defined. The posterior ears comparatively 

 large, well defined, the posterior border falcate. 



Interior. — Unknown. 



Exterior. — The surface is ornamented with numerous radiating ribs, closely set 

 with imbricating scales ; and between each pair smaller ones arise, with here and 

 there certain ribs stronger than others. These ribs are crossed occasionally by 

 concentric lines of growth. The posterior ears and left anterior ear are crossed 

 by numerous radiating ribs; the right anterior ear is crossed by three strong rib's, 

 but has concentric lines of growth strongly marked. 



Dimensions. — PI. XIV, fig. 11, a left valve, measures — 



Antero-posteriorly . . . .30 mm. 



Dorso-ventrally . . . .28 mm. 



Localities. — England: Black Limestone of Derbyshire; above the Underset 

 Limestone, Farcote Grill, Westmoreland. Scotland: Carboniferous Limestone 

 series of "West Quarry, Salton; Cateraig Sea Quarry, Dunbar; Carrielee Quarry, 

 Tynewater. Ireland : Arenaceous Limestone of Knockonny, Ballygawley, co. Tyrone. 



Observations. — M'Coy's types are probably preserved in the Griffith Collection, 

 Museum of Science and Art, Dublin, where I have been permitted to examine 

 them. The specimens are not good and have been crushed, but still the characters 

 of the species are fairly definite and can be recognised. M'Coy's account is 

 somewhat confusing: — "Valves dissimilar, the one marked with close, fine, smooth, 

 radiating striae, slightly flexuous, and nearly equal in size ; the other radiated with 

 about twelve larger, rounded ribs, each having a very fine sharp ridge on each 

 side, each set of ribs being separated from the rest by a narrow Hat space; all the 

 radiating ridges on this valve are closely set with small, imbricating, scale-like 

 laminae of growth." lie does not say which valve has the special ornament 

 mentioned. There are three specimens in the collection, and neither on them nor 

 in the figure can I distinguish the ornament described above. The tact is that, as 

 in most of the species of this size, there is much irregularity in the growth 

 and size of individual ribs, and in the majority of ribbed Pectiniform shells the 

 large or primary rib, which starts from the umbo, is associated with a narrow 

 secondary rib on each side. 



