PARALLELODON THECIFORMIS. 171 



Parallelodon theciformis, de Koninck, 1885. Plate XI, figs. 11 — 16. 



Paeallelodon theciformis, de Koninck, 1885. Ann. Musee Eoy. Hist. Nat. 



Beige, vol. ix, p. 158, pi. xxiv, figs. 26, 27 (The- 

 cidiformi8 in the explanation of plate xxiv.) 

 Cf. — pinguis, de Koninck, 1885. Ibid., p. 159, pi. xxiv, fig. 38. 



Specific Characters. — Shell of very medium size, transversely elongate, 

 subrhomboidal, moderately gibbose. The anterior end is short, and equal to or 

 greater in its dorso-ventral diameter than the posterior extremity. Its border is 

 almost straight at the anterior-superior angle, which is a right angle, and forms 

 the most anterior portion of the shell ; the margin very soon becomes semi- 

 circularly curved, and passes into the ventral border. The latter is straight in 

 its middle portion, and curved upwards at each end. The posterior end is 

 truncate, and slightly oblique from above downwards. The posterior-inferior 

 angle is bluntly curved, and the posterior-superior angle is an obtuse angle. The 

 hinge-line is straight, and the cardinal border is the longest part of the shell. 

 The umbones are tumid, slightly raised above the hinge-line, curved forwards and 

 inwards, situated in the anterior quarter of the hinge-line, separated from each 

 other by a well-marked ligamental facet. The valves are evenly and regularly 

 convex, but compressed at the dorsal slope, so as to make this part of the shell 

 slightly hollow. There is no oblique keel or ridge. The interior is quite 

 unknown. 



Exterior. — The surface is ornamented with fine concentric lines of growth 

 which may become somewhat separated on the posterior slope. There are no 

 radiating lines ; but at times, under the microscope, a fine granulation seems to 

 be the commencement of such an ornament. 



Dimensions. — PI. XI, fig. 11, measures — 



Antero-posteriorly . . . .18 mm. 



Dorso-ventrally .... 9'5 mm. 



Elevation of valve . . . .5 mm. 



Locality. — The Carboniferous Limestone of Settle, Yorkshire ; Castleton, 

 Derbyshire. 



Observations. — I have referred a series of six shells in the Burrow Collection 

 of the Woodwardian Museum, Cambridge, to this species of de Koninck, founded 

 by him on a single specimen from the Limestone of Vise. Only one of the 

 specimens, PI. XI, fig. 12, approaches the size of the type shell, but I feel 

 satisfied with the correctness of my reference. The absence of radiating strige, 

 and the greater comparative size of the anterior end in a dorso-ventral direction, 

 I regard as characteristic features. 



