352 CARBONIFEROUS LAMELLIBRANCHIATA. 



Cypkicardella crebricostata, Armstrong, sp., 1865. Plate XXXIX, figs. 12, 13. 



Ctpricardia crebricostata, Armstrong, 1865. Trans. Geol. Soc. Glasgow, vol. ii, 



pt. 1, p. 28, pi. i, fig. 4. 

 Young and Armstrong, 1871. Ibid., vol. iii, Supple- 

 ment, p. 50. 



— — Young, Armstrong, and Robertson, 1876. Cat. 



Western Scottish Fossils, p. 53. 



— — -Bigsby, 1S78. Thesaurus Devonico-Carboniferus, 



p. 203. 

 — Etheridge, 1S88. Brit. Foss., pt. 1, Palaeozoic, 



p. 282. 



Specific Characters. — Shell transversely subquaclrate, tumid, anterior and pos- 

 terior ends both narrowed in the dorso-ventral direction, inequilateral. The anterior 

 end is short, the antero-superior angle a well-marked right angle, from which 

 the margin descends, becoming rounded below, and passes with a rapid curve into 

 the inferior border. The upper part of the anterior end is much encroached upon 

 by a large depressed lunule. The inferior border is almost straight, rounded at 

 either extremity. The posterior margin is bluntly truncate and almost straight, 

 somewhat narrowed in adult specimens by the depression of the posterior part of 

 the hinge-line. The hinge-line is arched, depressed and extended posteriorly, 

 and may be emarginate, thus causing the postero-superior angle to vary in degree. 

 The postero-inferior angle is a right angle. The umbones are comparatively 

 large, tumid, elongate, twisted inwards, pointed, contiguous, very little raised, and 

 placed in the anterior third of the valve. Passing obliquely downwards and 

 backwards from the umbo to the postero-inferior angle is a bluntly rounded ridge, 

 which separates the compressed and hollowed dorsal slope from the rest of the 

 valve. The upper margin of the shell is formed behind by a raised angular everted 

 ridge, which passes backwards and is somewhat depressed, and which separates 

 the deep, wide and elongated escutcheon from the dorsal slope. Anterior to the 

 oblique ridge the valve is regularly and convexly curved from above downwards 

 and before backwards, being more convex in the dorso-ventral diameter. 



Interior. — Not yet seen. 



Exterior. — The surface is ornamented with very numerous regular distinct 

 rounded ridges, only well seen under the microscope, which are continued across 

 the dorsal slope to the upper margin of the valve. Shell thin. 



Dimensions. — Fig. 13, PI. XXXIX, from shale above the Arden Limestone, 

 in the collection of Mr. J. Smith, measures — 



Antero-posteriorly . . . .12 mm. 



Dorso-ventral ly . . . .10 mm. 



From side to side . . . 7 mm. 



