112 CARBONIFEROUS LAMELLIBRANCHIATA. 



the anterior ears, triangular, the posterior superior angle being only very slightly 

 obtuse. Shell thin. 



Interior. — Smooth. Muscle-scar in normal situation. Hinge not known. 



Exterior.- — Both valves are adorned with regular, concentric, flattened ridges, 

 separated by shallow, linear, concentric grooves, which are much closer together 

 and more numerous on the left than on the right valve, and also are crowded and 

 less regular near the lower margin. On the left valve the concentric ridges are 

 crossed by fine radiating lines, most marked at the anterior border. The anterior 

 ears have fine radiating and concentric ridges, while the posterior ears are almost 

 smooth. 



1) intensions. — PI. XX, fig. 10, a left valve, measures — 



Antero-posteriorly . . . .50 mm. 



Dorso-ventrally . . . .48 mm. 



Elevation of valve . . . .7 mm. 



Localities. — England : the Carboniferous Limestone of Wetton, Staffordshire ; 

 Castleton, Derbyshire. Ireland : the Carboniferous Limestone of Askeaton, co. 

 Limerick. 



Observations. — The marking of the right valve might be mistaken for that of 

 A. dissimilis if the ears were not preserved, but the latter shell is much flatter and 

 not so circular. The hinge-line and ears are totally different in character. The 

 species is not common, but has a fairly wide distribution. The valves seem to be 

 equally convex, but as I have not obtained a bivalved example, I cannot be certain 

 on this point. The shell was very thin, and if the specimens are at all rolled the 

 characteristic concentric lines and spaces are obliterated. The ears and the length 

 of the hinge-line show the real generic affinity. The general form and ornament 

 have some resemblance to the Cretaceous Pecteu ductus, Sow. 



Genus Crenipecten, Hall, 1883. 



Crenipecten, Hall, 1883. Pal. N. York, vol. v, pt. 1, plates and exp., p. 3. 

 _ _ 1885. Ibid., vol. v, pt. 1 ; Lamell., vol. i, p. 81. 



Generic Characters. — Shell Pectiniform, ears well developed, smooth, the 

 anterior ear the larger (Hall). Hinge with a number of small, close, subequal, 

 parallel, vertical, ligamental pits or teeth throughout its entire length. Surface 

 smooth or radiated. 



Observations. — This genus was founded by Hall for shells possessing the peculiar 

 hinge-characters described above. It differs from Pernopecten, Winchell, which 



