CRENIPECTEN SEMICIRCTTLARIS. 113 



lms the shape of Syncyclonema and Amusium, but lias a central cartilage pit and a 

 row of vertical, narrow pits, which are arranged, not in a straight line, but each 

 series rises slightly from the centre outwards. It also differs from Euchondria, 

 Meek, which has an excentric cartilage pit, and the lateral pits unequal on the 

 two sides. 



All the species figured by Hall, except one, G. Winchelli, are smooth, with a 

 short hinge-line and small ears ; but the species just mentioned has a long hinge- 

 line, large ears, and well-marked, numerous, radiating ribs. The only British shell 

 which shows the characteristic hinge-plate of Grenvpecten resembles the latter 

 species very closely. Prof. Hall says of it : — " This species differs from any known 

 form of Grenvpecten. It is like Aviculopecten in external form and surface 

 characters, and resembles the recent genus Pecten . . . Except in the hinge cren illa- 

 tions, this form has no other relations with the genus Grenipecten, and it may be 

 found to belong to some genus yet undescribed, which Avill include other species, 

 now placed with tho Carboniferous forms of Aviculopecten." 



Ceenipecten semiciecularis, M'Coy, sp., 1844. Plate XV, figs. 21—23. 



Pecten semicirculaeis, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 99, 



pi. xvii, fit;'. 10. 



Specific Characters. — Shell of medium size, almost equivalve, the valves 

 moderately convex, transversely semicircular. The margin regularly rounded, 

 extending in front of the anterior ear, markedly falcate posteriorly where it joins 

 the posterior ear. The hinge-line straight, long, projecting beyond the posterior 

 margin. The umbones small and inconspicuous, not raised, central. The anterior 

 ears depressed and well defined from the body of the valve, that of the right valve 

 slit for the byssus ; the posterior ears long, large, and well defined, the posterior 

 border very falcate. 



Interior. — Smooth. 



Exterior. — The surface of the right valve is ornamented with very numerous, 

 fine, somewhat irregular, rounded ribs on the body of the valve, totally absent on t he 

 posterior ear, and with only five on the anterior ear. These ribs are crossed by 

 concentric lines and rugae of growth, which are well marked on the posterior ear. 

 The same ornament occurs on both valves. 



Dimensions. — PL XV, fig. 21, a left valve, measures — 



Antero-posteriorly . . . . 72 mm. 



Dorso-vent rally . . . .01 mm. 



