PROTOSCHIZODUS SUBTRUNCATUS. 249 



gibbose. The anterior portion comprises about one-third of the valve, is deep, 

 and has its border semicircularly curved. The inferior border is regularly curved 

 in front, continuously with the anterior edge ; but behind, where it joins the 

 posterior border, the curvature is much more rapid, and there is no approach to 

 angulation. The posterior border is subtruncate, but more or less convex ; and 

 it joins the superior border with an obscure, almost obsolete, obtuse angle. The 

 hinge-line is gently curved. The umbones are small, triangularly pointed, not 

 much raised above the hinge-line, contiguous, anterior, and slightly twisted 

 forwards. The valve is rapidly compressed posterior to a line passing obliquely 

 downwards and backwards from the posterior edge of the umbo, so that the 

 dorsal slope is concave; but there is no ridge or approach to angulation. 



Interior. — The muscle-scars are normal in position. Hinge not yet exposed. 



Extertor. — The surface is almost smooth, but under the microscope very 

 faint concentric lines of growth are visible. Shell very thin. 



Dimensions. — Fig. 4, PI. XX, the type of Amphidesma subtruncata, M'Coy, 

 measures — 



Antero-posteriorly . . . .33 mm. 



Dorso-ventrally . . . .30 mm. 



Elevation of valve . . . .11 mm. 



Localities. — England: the Carboniferous Limestone of Park Hill, Derbyshire. 

 Ireland : the Carboniferous Limestone of Millicent, Clane, co. Cork ; and Firogh, 

 co. Limerick. 



Observations. — The figure given by M'Coy of his Amphidesma subtruncata is 

 very different from the original shell. As is the case with all M'Coy' s figures, 

 the original, a right valve, is depicted as a left ; and the shell is more oblique, and 

 not so quadrate as represented in the figure. That the specimen which I 

 reproduce, PL XX, fig. 4, now preserved in the Griffith Collection in the 

 Museum of Science and Art, Dublin, is the orginal, there can be no doubt, the 

 contour of the line which shows where the shell is absent being identical with 

 that shown in the original drawing. The type is a little imperfect in front, but 

 otherwise is a very good specimen. Unfortunately I have not been able to 

 obtain any other examples but the three fragmentary specimens here figured, 

 PI. XX, figs. 1 — 3, which are from the Limestone of Park Hill, Derbyshire. 

 De Koninck figures a single example of this species from Pauquys, near Dinant, 

 and states that it is not so very rare in one locality, but that he has only seen a 

 single complete specimen. This author confounds this species with the Dolabra 

 seqiiilateralis, M'Coy (now re-described as P. sequilateralis), placing the latter as 

 a synonym of P. subtruncatus. For some time I thought that this view was 

 correct, but P. sequilateralis has a much more truncate posterior end, its umbones 

 more anterior, and marked off both in front and behind by distinct umbonal 



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