AVICULOPHCTUN DKCUSSATUS. 79 



marked " with about ten or twelve sharp radiating striae similar to those of the body, 

 and crossed by very delicate strise." 



Aviculopecten DECUSSATUS, M'Coy, sp., 1844. Plate XVIII, figs. 19, 20. 



Lima decussata, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 87, pi. xv, fig. 3. 

 Cf. — concinna, M'Coy, 1844. Ibid , p. 87, pi. xv, fig. 6. 

 Cf. Pecten tripaktitus, M'Coy, 1844. Ibid., p. 101, pi. xvi, fig. 8. 



Specific Characters. — Shell below medium size, ovate, acute, moderately gibbose. 

 The margin regularly rounded. The hinge-line straight, of moderate length. 

 Umbones small, pointed, subcentral. Ears well defined ; the anterior large, 

 depressed, deep, and rectangular ; the posterior small and triangular. 



Inter io r. — Unknown . 



Exterior. — The surface is ornamented with very close, fine, radiating lines, best 

 seen on the sides in the young, but present all over the lower part of the valve. 

 The ears have the same ornament decussated by concentric lines of growth. 



Dimensions. — PI. XVTII, fig. 19, the type of Lima decussata, M'Coy, measures — 

 Antero-posteriorly . . . .13 mm. 



Dorso-ventrally . . . .16 mm. 



Localities. — Ireland : the Carboniferous Limestone of Killymeal, eo. Tyrone ; 

 Ballinglen, co. Mayo. 



Observations. — The types of L. decussata, L. concinna, and Pecten tripartitus are 

 all present in the Griffith Collection, Museum of Science and Art, Dublin. The 

 first shell is puzzling, for while it is a left valve, incomplete on the posterior 

 edge of the valve, the figure shows a whole shell ; but the description, on the other 

 hand, reads — "Obliquely ovate, . . . ears very small, equal; hinge-line oblique; 

 surface radiated with numerous, unequal, obtuse, smooth ridges; the spaces 

 between eaeli are strongly striated transversely." The figure and type are not 

 oblique, have each unequal, rather large ears, a straight horizontal hinge-line, and 

 I cannot detect any transverse striations. I think, therefore, M'Coy's description 

 must be disregarded, and as figure and specimen agree fairly well, it may be 

 assumed to be the original type. 



I think it probable that /,. conriiuia, M'Coy, was a juvenile form of the same 

 species, but I have no doubt that P. tripartitus, M'Coy, is a very young example of 

 .1. drcnssafiis, and that the peculiar arrangement of the radiating lines, supposed to 

 be typical of P. tripartitus, is either accidental or due to an early condition of 

 growth. 



