92 CARBONIFEROUS LAMELLIBRANCHIATA. 



Specific Characters. — Shell small, oblique anteriorly, ovate, acute, the left valve 

 convex, the right less so. The margin rounded, the curvature starting at a lower 

 point in front than behind owing to the larger size of the anterior ear, which is 

 much and rapidly depressed from the body of the valve. The hinge-line straight, 

 of moderate length. The umbones triangular, pointed, convex, central. The 

 anterior ear of the right valve separated from the rest of the shell by a slit for the 

 byssus. The posterior ear small, rapidly compressed, and triangular. 



Interior. — Unknown. 



Exterior. — The left valve is ornamented by close concentric lines and radiating 

 ribs of almost the same strength, giving a fine reticulated appearance. The ears 

 seem to possess somewhat the same character, only less well marked. The 

 ornament of the right valve is not essentially different from that of the left valve. 



Dimensions. — PL XV, fig. 11, measures — 



Antero-posteriorly . . . .15*0 mm. 



Dorso-ventrally .... 15'0 mm. 



Convexity of left valve .... 2*5 mm. 



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

 Poolvash, Isle of Man. Scotland: Buriebrae Burn, near Milton of Campsie; Annick 

 Water, near Stewarton. Ireland : the Carboniferous Limestone of Little Island, 

 co. Cork ; Bundoran, co. Donegal. 



Observations. — This species was founded on a single imperfect example from 

 which the ears were wanting, and it is impossible to say which valve it is. The 

 type is preserved in the Griffith Collection, Museum of Science and Art, Dublin. 

 The cabinet of Mr. Joseph Wright contains six good examples of A. planoclathratus 

 from the Limestone of Little Island, co. Cork, four of which are right and three left 

 valves. The anterior ears are well preserved in all the specimens, but the posterior 

 are lost. The characters of the shell are very marked, and I have no hesitation in 

 retaining the species. 



I think it possible that Aviculopecten tenuilineatus, de Koninck, may represent 

 A. jilanoclathrat/us. Its antero-posterior and dorso-ventral diameters are the same, 

 and its ornament is similar ; but the specimen figured is much larger than any I 

 have yet seen. In the enlarged view of the surface the concentric lines are shown 

 to be less strongly marked than the radiating ribs. I am unable to see by what 

 essential characters A. tenuilineatus is distinguished from A. perplicatus, de 

 Koninck, the latter being only a larger shell. 



