PARALLELODON DECUSSATUS. 159 



radiating ribs on the dorsal slope, and the very fine almost microscopic radiating 

 lines over the rest of the shell, best seen in front and near the lower margin, both 

 of which characters are absent in the latter species. The type specimen comes 

 from the Calp or Middle Limestone series of Manorhamilton, the horizon 

 of which is seen, on reference to the general sequence of the Carboniferous rocks 

 of Ireland given at page 14. In the neighbourhood of Dublin, however, it is 

 impossible to draw a line of demarcation between the Middle Limestone or Calp 

 and the Upper Limestone. It is noteworthy that the species also occurs in beds 

 near Kilkee, Co. Clare, which are classed as Coal-measures by the Irish Geological 

 Survey. The specimen is in the Survey Collection at the Science and Art 

 Museum, Dublin. 



Fig. 10, PI. XI, has not come out well in the drawing. It is the cast of the 

 interior, and does show some of the details of the posterior part of the hinge, 

 but they are hidden behind the cast of the umbo, and therefore could not be 

 depicted by the artist. 



Parallelodon deoussatus, M'Coy, sp., 1844. Plate XI, fig. 22 ; Plate XII, figs. 11, 



12, and 15. 



Psammobia becussata, M'Coy, 1844. Synopsis Carb. Foss. Ireland, p. 53, pi. x, 



fig. 2. 

 Aeca akatina, d'Orbigny, 1850. Prodrome Pal., vol. i, p. 134. 



— — de Koninck, 1851. Anim. Foss. Terr. Carb. Belgique, Supplement, 



p. 671, pi. lvii, fig. 7. 

 Psammobia decussata, Morris, 1854. Cat. Brit. Foss., p. 222. 

 Aeca anatina, Bigsby, 1878. Thesaurus Devonico-Carboniferus, p. 299. 



— decussata, Bigsby, 1878. Ibid., p. 299. 



— — Etheridge, 1885. Brit. Foss., part 1, Palseoz., p. 279. 

 Paballelodon decussatus, de Koninck, 1885. Ann. Musee Eoy. Hist. Nat. 



Beige, torn, xi, p. 154, pi. xxv, fig. 29. 



Specific Characters. — Shell of moderate size, obliquely sub-ovate, moderately 

 gibbose. The anterior end is short and somewhat swollen, its border is 

 straight for a short distance above, where it joins the hinge-line at a right angle ; 

 it soon becomes semicircularly curved, and passes into the inferior border, which 

 is also regularly curved, and convex downwards. The posterior border is obliquely 

 truncate from above downwards, and straight, joining the inferior and superior 

 borders at obtuse angles, the lower one being rounded. The hinge-line is straight 

 and somewhat shorter than the longest diameter of the shell. The umbones 

 gibbose, elevated above the hinge-line, incurved, very close, separated only by a 

 linear ligamental area. 



