PROTOSCHIZODUS NUCULOIDES. 243 



Dimensions. — Fig. 11, PI. XVIII, measures — 



Antero-posteriorly . . . .25 mm. 



Dorso-ventrally . . . .26 mm. 



From side to side . . . .12 mm. 



Locality. — Scotland : Ayrshire, the shale under the Main Limestone, Beith, 

 Lower Limestone series. 



Observations. — To Mr. R. Craig of Beith I am indebted for the kind gift of 

 the shell on which this species is founded, and which I am unable to identify 

 with any other form yet described. He tells me that this is the shell generally 

 known in the west of Scotland as Axinus carbonarius, Portlock, now described as 

 Protoschizodus axiniformis, var. de/pressus, Portlock, var. The latter species is not 

 so regularly triangular or so nearly equilateral, and has a well-marked oblique 

 ridge and dorsal slope, which characters may be relied upon for the differential 

 diagnosis of the two species. One of the specimens of P. Cantrainianus, 

 de Ryckholt, sp., in the Museum of Natural History, Brussels, somewhat 

 resembles this species, but the figured specimen is totally unlike ; it is more 

 oblique, and has the dorsal slope more developed. 



Pbotoschizodus nuculoides, M'Coy, sp., 1844. Plate XIX, figs. 10 — 16. 



Axinus nuculoides, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 63, pi. xi, 



fig. 9. 

 Anatina deltoidea, M'Coy, 1844. Ibid., p. 51, pi. viii, fig. 7. 

 Axinus? nuculoides, Morris, 1854. Cat. Brit. Foss., 2nd edit., p. 189. 

 Anatina? deltoidea, Morris, 1854. Ibid., p. 183. 



Schizodus nuculoides, Etheridge, 1888. Brit. Foss., vol. i, Palseoz., p. 290. 

 Anatina deltoidea, EtJieridge, 18SS. Ibid., p. 277. 



Specific Characters. — Shell small, ovate, transverse, moderately convex, bluntly 

 pointed behind. The anterior end comprises about one third of the valve, and is 

 comparatively deep in a dorso-ventral direction ; its border is gradually curved, 

 and not well defined either above or below. The inferior border is extended, only 

 very gently convex for the greater part, but more so posteriorly, where it joins 

 the posterior border, making an elliptical curve. The posterior border is long, 

 oblique, almost straight, descending backwards and downwards. The hinge-line 

 is fairly extensive and gently arcuate, passing behind into the posterior border 

 without a break. The umbones are small, pointed, forming the largest portion of 

 the valve in front ; the anterior umbonal slope is not well defined, but proceeding 

 downwards and backwards towards the postero-inferior angle is a well-marked 



