PARALLELODON OBTUSUS. 167 



Paeallelodon obtusus, Phillips, sp., 1836. Plate VIII, fig. 9 ; Plate XII, figs. 



2—6, and 9 ; Plate XIII, fig. 7 ; Plate 

 XIV, figs. 1—3. 



CuctrLL-ffiA obtusa, Phillips, 1836. Geol. Yorks., vol. ii, p. 210, pi. v, fig. 19. 



Abca obtusa (pars),de Koninck, 1842. Anim. Toss. Terr. Carb. Belgique, p. 112, 



pi. ii, figs. 15 c and d. 



Byssoabca obtusa, IP Coy, 1844. Synopsis Carb. Poss. Ireland, p. 73. 



Abca obtusa, Bronn, 1844. Nomencl. Palaeont., p. 97. 



Cucull-Da obtusa, Brown, 1849. Ulust. Poss. Conch. G-t. Brit., p. 194. 



Abca obtusa, cPOrbigny, 1850. Prodrome paleont., pt. i, p. 134. 



Cucull.ea obtusa, Morris, 1854. Cat. Brit. Foss., p. 197. 



Byssoabca obtusa ?, Griffith, 1860. Journ. Geol. Soc. Dublin, vol. ix, p. 93. 



Cuculljea obtusa, Bigsby, 1878. Thesaurus Devonico-Carboniferus, p. 305. 

 ? Paeallelodon obtusus (pars), de Koninck, 1885. Ann. Musee Hist. Nat. Beige, 



vol. xi, p. 147, pi. xxiv, figs. 6, 14 — 

 20, 22, 34, 35. 



Abca obtusa, Etheridge, 1885. Brit. Poss., part i, Palaeoz., p. 279. 



Specific Characters. — Shell of moderate size, transversely oblong, gibbose, with 

 subparallel borders. The anterior end of the shell is compressed and narrowed 

 in its dorso-ventral diameter, and has a very pronounced anterior-superior angle, 

 which is a right angle. The anterior border descends for a short distance in a 

 straight line, then becomes rapidly and elliptically curved, and passes into the 

 inferior border. This is almost straight for the greater part of its length, and 

 passes into the posterior border with a broadly rounded curve. This is straight 

 and obliquely truncate from above downwards and backwards, making a well- 

 marked obtuse angle with the hinge-line posteriorly. The hinge-line is straight, 

 a little shorter than the transverse diameter of the shell. The umbones are large, 

 tumid, curved inwards and forwards, separated, much raised above the hinge- 

 plate, and situated in the anterior fourth of the hinge-line. 



The umbonal swelling is well marked off in front from the anterior end, though 

 it rises gradually from the body of the valve. The valve is arched from above 

 downwards, so that the crown of the arch is at about the centre of the dorso- 

 ventral curve. Behind an oblique line passing from the posterior edge of the 

 umbo to the posterior-inferior angle, the valve becomes rounded off into the 

 dorsal slope, and is compressed and flattened towards the posterior-superior angle. 

 The ligamental area is large, wide in front, and gradually narrowing till it becomes 

 obsolete towards the posterior end ; it is striated longitudinally for the attachment 

 of the external ligament. 



Interior. — I have seen no specimens showing the position of the adductor 

 muscles. The hinge-plate is thickened in front, on which are traces of several small 



