56 
DEVONIAN FAUNA. 
seen that hardly two can be found alike, and that they pass one into another by 
imperceptible differences. It is one of the commonest bivalves at Lummaton. 
It seems to have been usually mistaken for and labelled Megalodon cucullatus, 
Sowerby, but from that shell it is totally distinct. Mr. Roberts and I carefully 
compared it with the figures and descriptions of the latter species given by 
Sowerby, Goldf uss, and Phillips, and with German specimens in the Woodwardian 
Museum, and we found them to be in every case utterly unlike, and not even 
to belong to the same genus. 
On my showing it to Mr. R. Btheridge, F.R.S., he suggested Gardiomorpha as 
possibly its genus ; but the numerous species figured by de Koninck seem to show 
that it differs essentially from that genus, especially in the much less erect and 
simple character of its umbo. On the other hand, as far as can be judged from 
external characters, it meets all the requirements of the genus Butotia ; and one of 
the Carboniferous species, Butotia ovalis, de Koninck,^ comes so close that there 
is great difficulty in differentiating it, considering the great variability of our shell. 
However, de Koninck's two figures are much smaller than our full-grown shells, 
while they are very unlike our young specimens of the same size. Their anterior 
wing also seems to be higher and more angular than usual, and their umbones 
rather blunter. On the whole, though they ultimately may prove identical, the 
Belgian evidence seems too slight at present to justify us in uniting shells which 
come from different formations. 
Affinities. — Unio castor, Bichwald,^ seems to be a deeper and narrower shell, 
with a more arched and incurved umbo and narrower wings. 
Myalina tenuistriata, Sandberger,^ closely resembles young specimens in shape, 
but is distinctly radiated. 
Pterinea arduensis, Steininger,* has definite front and hind wings, and numer- 
ous concentric strige. 
Inoceramus regidaris, Miinster,'^ which is the same as Posidonomya regidaris, 
Geinitz,^ is larger and flatter, and has some concentric striee and less distinct 
wings. 
Inoceramus semistriatus, Miinster,'^ bears radiations on the anterior side. 
Posidonomya {?) grandis, Miinster,^ is an imperfectly described species which, 
though much larger, has some likeness to the more equilateral varieties of our 
shell, but does not show sufficient characters for full comparision. 
1 1885, de Koninck, ' Ann. Mus. Eov. H. N. Belg.,' vol. ix, pt. v, p. 200, pi. xxii, Ggs. 36, 40. 
2 1860, Eichwald, ' Lethjea Eossica, p. 1003, pi. xxxix, figs. 20 a, I. 
3 1868, Sandberger, ' Verst. Ehein. Nassau,' p. 280, pi. xxix, figs. 10, 10 a. 
4 1853, Steininger, 'Geogn. Beschreib. Eifel,' p. 55, pi. ii, figs. 8—11. 
5 1840, Miinster, ' Beitr.,' pt. 3, p. 48, pi. x, figs. 1 a, h. 
^ 1853, Greinitz, ' Verst. G-rauw. Sachsen,' pt. 2, p. 51, pi. xiii, figs. 3, 3«, 4, 4«. 
' 1840, Miinster, ' Beitr.,' pt. 3, p. 49, pi. x, figs. 7 a, h. 
8 Ibid., p. 50, pi. x, fig. 10. 
