SOLENOMORPHA. 157 



obliquely truncate, nearly straight, but at times formed of a longer upper and 

 a shorter lower portion which meet at a very wide angle. The postero-inferior 

 angle is well marked and is almost a right angle. The postero-superior angle 

 is very little larger than a right angle. Hinge-line straight, shorter than the 

 ventral border. The umbones are small, pointed, incurved, and twisted forwards, 

 slightly raised above the hinge-line, and placed in the anterior quarter of the 

 valve. Proceeding obliquely outwards and downwards from the umbo to the 

 postero-inferior angle is an oblique, angular ridge, dividing the valve into two 

 unequal portions ; the upper and smaller forms the dorsal slope, which is much 

 compressed, and bisected by a radiating line. The inferior part corresponds to 

 the body of the valve and is compressed by a broad, shallow, oblique sulcation, 

 which is indicated at the inferior margin by a sinus. Lunule and escutcheon 

 elongate and narrow. 



Interior. — The adductor muscle-scars are normal in position. Hinge-plate 

 slightly arched. Edentulous. 



Exterior. — The surface is ornamented with several coarse, somewhat irregular 

 concentric lines and striae of growth. 



Dimensions. — PL XXIII, fig. 18, a left valve, measures — 



Antero-posteriorly . . . .23 mm. 



Dorso-ventrally . . . .11 mm. 



Gibbosity of valve . . . .3 mm. 



locality. — The Upper Limestone Series of Linn Spout, Dairy. 



Observations. — 8. vexillum is not uncommon in the shales beneath the Linn 

 Spout Limestone, Dairy. I have been able to expose the hinge-plate of both 

 valves and find it edentulous. This species has some resemblance to 8. striato- 

 lamellosus, of which I suggested (supra, vol. i, p. 398) it was a synonym ; but the 

 latter is more rugged and a much larger shell, and has the posterior border poly- 

 gonal, and I think it advisable to retain de Koninck's specific name. De Koninck 

 places S. vexillum in a group whose surface has three or four diagonal folds, but in 

 the diagnosis describes one main fold and a much less well-marked secondary 

 ridge. I find the species to agree with specific description rather than with that 

 of the group. De Koninck states that he found the species at Craig, Scotland. 

 The hinge-plate is exposed in both valves (PI. XXIII, figs. 17a, 18^). 



Genus Solenomorpha, Hind, 1903. 



Solen, pars, Goldfuss, 1832. H. von Dechen's transl. of the 2nd. ed of tic la Beche's 

 Manual of Geognosy, p. 531. 

 — Goldfuss, 1840. Petrefacta Germanise, vol. ii, p. 276. 

 Pnrllock, 1843. Rep. Greol. Londonderry, etc., p. 441. 



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