82 CARBONIFEROUS LAMELLIBRANCHIATA. 



Aviculopecten clatheatus, M'Coy, sp., 1844. Plate XV, figs. 1 — 7. 



Pecten clatheatus, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 90, pi. xiv, 



fig. 12. 

 — intercostatus, M'Coy, 1844. Ibid., p. 95, pi. xviii, fig. 4. 



Specific Characters. — Shell of medium size, slightly oblique, transverse, tri- 

 angularly fan-shaped, moderately gibbose, and very inequi valve, the right being 

 almost as flat as the left valve. The ears, especially the anterior, small and 

 depressed. The hinge-line short and straight. The lower margin broadly convex, 

 and the anterior and posterior margins almost straight, inclined obliquely inwards 

 and upwards to the base of the ears. The umbo in the left valve small, pointed, 

 SAvollen, and incurved, subcentral, that in the right valve almost obsolete. 



Interior. — Unknown. 



Exterior. — The left valve is ornamented with from fifteen to eighteen strong 

 radiating ribs, and between each pair are three other finer ribs, of which the 

 centre is the stronger and extends further towards the umbo. It may happen 

 these secondary strong ribs become so well marked that the valve appears to have 

 alternate strong and weak ribs. The valve is also crossed by close concentric lines, 

 which are best marked in the spaces between the ribs and on the surface of the 

 finer ribs. The ears have radiating ribs and concentric strias. The right valve is 

 smooth both on the bod}?- and on the ears. 



Dimensions. — PI. XV, fig. 1, a specimen in the collection of Mr. Joseph Wright, 

 measures — 



Antero-posteriorly . . . .27 mm. 



Dorso-ventrally . • • .23 mm. 



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

 Narrowdale, Staffordshire ; Settle, Yorkshire. Ireland : the Carboniferous Lime- 

 stone of Little Island and Ballydaniel, co. Cork ; Drumkeeran, co. Fermanagh. 



Observations.- — This species was founded on the peculiar ornament of a left valve. 

 The right valve proves to be quite smooth, as is indicated by a very young 

 bivalved example in Mr. Joseph Wright's collection, from Little Island, co. Cork 

 (PI. XV, fig. 4). The peculiar marking described by M'Coy is only well developed 

 in the full-grown shell. As the primary ribs, starting from the umbo, pass across 

 the valve, one new rib arises between each pair, and some little way further on 

 another rib becomes developed between each primary and secondary. If the 

 secondary ribs are well developed there is a condition of alternate large and small 

 ribs, the normal condition near the umbo; but occasionally all three sets of ribs 

 remain a different size, which is the condition described by M'Coy. 



