NUCULA iEQUALIS. 189 



Exterior. — The surface is almost smooth ; but under the microscope very fine 

 and regular concentric lines, with occasionally, towards the lower margin, a more 

 conspicuous concentric fold or hollow. 



Dimensions.— The type Specimen Antero-posteriorly. Dorso-ventrally. Laterally. 



(PL XIV, fig. 23) measures — 17 mm. 6 mm. — 



PI. XIV, fig. 25 measures— 13 mm. 7 mm. 4 mm. 



Localities. — Scotland : Upper Limestone Series of Orchard and Waulkmill Glen, 

 Thornliebank. Ireland : Arenaceous shale, Monaduff, Drumlish. 



Observations. — This species, of very characteristic shape, seems to be very rare 

 indeed. I reproduce the type specimen, preserved in the Griffith Collection, 

 Science and Art Museum, Dublin (fig. 23, PI. XLV) ; and this, together with 

 a small series from the west of Scotland, collected by Messrs. John Smith and 

 James Neilson, are all that I have yet seen. 



M'Coy observes, " This shell is distinguished by its great transverse diameter, 

 subtruncate posterior end, and the parallelism of the hinge-line and abdominal 

 margin, the posterior end being as wide as any other part of the shell." 



Nucula .equalis, Sotverby, 1840. Plate XIV, figs. 32 — 35. 



Nucula .squalis, Sowerby, 1840. Trans. Geol. Soc, 2nd ser., vol. v, Appendix, 



Explanation of pi. xxxix, figs. 3, 3 a, 3 b. 



— — Brown, 1849. Illustr. Foss. Conch., p. 187 (figure wanting). 



— liiteata, Armstrong and Young, 1871. Trans. Geol. Soc. Grlasg., vol. iii, 



Appendix, p. 53. 

 Ctenodonta ^qualis, Bigsby, 1878. Thesaurus Devonico-Carboniferus, p. 303. 

 Nucula .equalis, Etheridge, 1888. Brit. Foss., part 1, Palseoz., p. 287. 



— lineata, Etheridge, 1888. Ibid., p. 287. 



Specific Characters. — Shell only moderately but evenly convex. The greatest 

 convexity is umbonal, regularly compressed into the borders, triangularly ovate ; 

 somewhat acute posteriorly. The anterior end is short and deep, and has a 

 regularly rounded border, passing with a regular semicircular curve into the 

 inferior border, which is regularly convex downwards. The posterior end is also 

 rounded, but its curve is more acute, owing to the descent of the posterior portion 

 of the hinge-line. The hinge-line is arcuate, or even angular ; a short anterior 

 and longer posterior limb, inclined to each other at a very wide angle, meeting 

 opposite the umbones. The umbones are moderately convex, incurved, twisted 

 forwards, and contiguous. 



The Interior. — The hinge-line consists of two series of closely set teeth, one 

 anterior and the other posterior, separated by a small pit or depression ; they are 

 inclined to each other at a very obtuse angle ; the anterior set small and few ; 

 those of the posterior set about six, increasing in size from before backwards. 



