MODIOLA LIGONULA. G9 



a distinct but small lunule. The anterior border, starting above at tlie most 

 anterior point of the shell, immediately descends with a regular curve posteriorly, 

 and passes imperceptibly into the inferior border, which is straight and directed 

 obliquely do"\vnwards and backwards. The posterior border is obliquely truncate 

 downwards and backwards in its upper half, bluntly rounded below, where it passes 

 with a regular curve into the inferior border. 



The hinge-line is slightly arched Avith convexity upwards, equal in length to 

 one-half the greatest oblique measurement of the shell. The umbones are tumid, 

 anterior but not terminal, pointed and directed forwards, contiguous. The posterior 

 or greater portion of the shell is obliquely swollen, the gibbosity extending down- 

 Avards and backAvards to the posterior inferior angle ; above the SAvelling the shell is 

 com|)ressed and expanded, being quite flat and thin at the posterior superior angle. 



Interior unknoAvn. 



Exterior. — The shell is almost smooth above, but very fine striae are visible, and 

 toAvards the ventral edge slight shalloAV folds, Avhich become fine lines at the posterior 

 border above. 



Dimensions. — PL II, fig. 13, measures — 



Dorso-A^entrally . . . . .10 mm. 



Antero-posteriorly . . . .16 mm. 



Elevation of single vah^e about . . _• 3 mm. 



Locality. — Carboniferous Limestone of Wensleydale. 



Observations. — I am, so far, aAvare of only one specimen of this species from 

 Great Britain, Avhich I liaA^e, Avith hesitation, referred to 21. emaciata of de Koninck, 

 described by J. Fraipont {pp. supra cit). A number of small species of Modiola 

 are described by these authors, Avhich are stated to differ from each other in series in 

 a feAv small details of size and shajDe. M. emaciata is said to resemble M. impressa, 

 de Koninck, more than any other, but this shell is " plus longue et moins haute 

 vers I'extremite posterieure." Both shells come from Vise, stage 3, and are said to 

 be very rare ; and it might turn out, if many specimens Avere obtained, that they Avere 

 only varietal forms of the same species. 



The shell figured is from the Manchester Museum, OAvens College, the authorities 

 of Avhich I have to thank for permission to describe the shell. 



Modiola ligonula, de Hyckholt, 1847. PI. VII, figs. 5, 5 a, 6. 



MXTiLUS LIGONULA, de Si/ckJwU, 1847. Mem. Couronn. Soc. Eoyale Belgique, 



vol. xxiv J Melanges Paleontologiques, part 1, 

 p. 136, pi. vii, figs. 18, 19. 

 — FABALis, de Eyckholt. Ibid., p. 137, pi. vii, figs. 20, 21. 



