PARALLELODON SEMICOSTATUS. 157 



Parallelodon semicostatus, M'Ooy, sp., 1843. Plate XI, figs. 5 — 10; Plate XIII, 



figs. 15, 15 a. 



Byssoabca semicostata, M'Goy, 1843. Synops. Carbonif. Foss. Ireland, p. 73, 



pi. xi, fig. 35. 

 Aeca semicostata, d'Orbigny, 1850. Prodrome Paleont., vol. i, p. 134. 

 Maceodok tenitistbiata, Meek and Worthen, 1867. Proc. Chicago Acad. Sci., 



vol. i, p. 17. 

 — — Meek, 1872. U.S. G-eol. Surv., Final Eep. Nebraska, 



p. 207, pi. x, figs. 20 a, b. 

 Aeca beticulata, Young and Armstrong, 1871. Trans. G-eol. Soc. Glasgow, 



vol. iii, p. 49. 

 — — — 1876. Cat. Foss. West. Scot., p. 49. 



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

 Maceodon tenuisteiata, Bigsby, 1878. Ibid., p. 426 d. 

 Aeca semicostata, Etheridge, 1885. British Fossils, vol. i, Pal., p. 280. 



Specific Characters. — Shell small, oblong, subrectangular, subcylindrical, 

 gibbose in comparison with its length. The anterior end is small, with its anterior- 

 superior angle pointed ; from this point the anterior border descends with a rapid 

 curve, and passes gradually into the inferior, which is almost straight, but 

 slightly curved upwards at each end. The posterior end is straight, obliquely 

 truncate from above downwards, making a rounded obtuse angle below, and an 

 angle only a little greater than a right angle with the upper and lower borders 

 respectively. The hinge-line is straight. The umbones are elongate, compressed 

 from side to side, incurved, non-contiguous, and elevated above the hinge-line, 

 from which they are separated by a fair- sized elongate ligamental area, which 

 becomes obsolete behind. The umbones are situated in the anterior third of the 

 shell. Passing downwards and backwards, and defining the posterior limit of 

 the umbonal swelling, is an oblique keel, which is acute above, but becomes less 

 and less angular and prominent till it terminates at the junction of the posterior 

 and inferior borders. Above and behind this ridge the shell is compressed rapidly, 

 so that it becomes slightly hollow. Anterior to the ridge the valve is swollen, and 

 there is a somewhat obscure, oblique byssal sulcus, a little anterior to the centre 

 of the shell. 



Interior. — The adductor scar is small, circular, situated at the anterior 

 superior angle, but remote from the anterior margin. The posterior adductor 

 scar is small, shallow, situated some little distance within the margin and above 

 the oblique depression corresponding to the ridge. The pallial sinus is entire and 

 remote from the margin. The anterior part of the hinge has several small oblique 

 teeth ; the posterior portion consists of three long, slightly diverging teeth. 



Exterior. — The convex surface of the valve is ornamented with fine strise and 



