PARALLELODON FALLAX. 161 



interior or the hinge, and have therefore quoted the hinge-characters from 

 de Koninck's description. 



The obliquely ovate form of this species is very characteristic, and marks off 

 the species distinctly from P. fallax, to which de Koninck thought it had some 

 resemblance. The latter shell is not oblique, and has a squarely cut posterior end ; 

 and in P. decussatus the umbones are better developed, and the anterior-superior 

 angle is less square, and tbe whole shell is more convex. 



I have with some hesitation referred three shells to this species, which I have 

 figured, PL XII, figs. 11, 12, and 15. They are from two different localities, and 

 are all imperfect specimens. Fig. 15 is in the Collection of the Geological Survey, 

 Jermyn Street, and was obtained from Narrowdale ; only a moiety of the shell is 

 left at the umbo and the dorsal slope ; in the latter position the microscope reveals 

 very fine, close-set, radiating and concentric lines. This fact, with the tumid 

 umbo and semicircular anterior end, has, I hope, warranted my reference. 



Of the two specimens, figs. 11 and 12 from Castleton, the former is a cast 

 and the latter a much worn testiferous example ; but the former has small portions 

 of the test preserved at the anterior border and the dorsal slope, which show, 

 especially in the latter position, the characteristic fine decussating lines of the 

 species. Unfortunately the type specimen is somewhat imperfect at the anterior- 

 superior angle, but the species is more ovate in form than any others of the same 

 genus, and the umbones are more regularly rounded and tumid than obtains as a 

 rule in Parallelodon, or even in the other genera of the Arcidge. 



Parallblodon fallax, de Koninck, sp., 1851. Plate XI, figs. 23 — 27 a, b ; 



Plate XII, fig. 14. 



Aeca fallax, de Koninck, 1851. Anim. foss. terr. carb. Belgique, Supplement, 



p. 672, pi. lv, figs. 6 a— c. 

 — — Bigsby, 1878. Thesaurus Devonico-Carboniferus, p. 299. 



Paeallelodon fallax, de Koninck, 1885. Ann. Musee Roy. Hist. Nat., 



torn, xi, p. 153 ; pi. xxv, figs. 4 — 9. 



Specific Characters. — Shell of moderate size, very inequilateral, equivalve, sub- 

 quadrate, convexly swollen. The anterior end is very short and compressed. 

 The anterior border meets the hinge-line at a right angle, and descending, at first 

 in a straight line, becomes semicircularly curved, and passes into the inferior 

 border without a break. The latter is regularly rounded, and meets the posterior 

 border at a rounded, slightly obtuse angle. The posterior border is truncated 

 and almost perpendicular, though in some examples the posterior-superior angle, 

 which is generally a right angle, is rounded off. The hinge-line is straight, and as 

 long as the greatest transverse diameter of the valve ; slightly elevated posteriorly 



21 



