NAIADITES MODIOLARIS. 131 
should have been bevelled at the expense of this upper surface, in order that the 
valves might open. 
The genus Naiadites was gregarious in its habit, and attached itself by the 
byssus to pieces of wood or other shells ; masses are often found together. 
There is in the British Museum (Nat. Hist.), Cromwell Road,’ an interesting 
slab from the shale above the Kinderscout grit of Rabchester, Lancashire, on 
which is shown a piece of wood surrounded on three sides by masses of Naiadites 
two and three deep, lying in many cases with their inferior borders towards the 
wood, as if attached to it by the byssus. This habit and the possession of a 
byssus precludes the idea that these shells were burrowers; and an additional 
piece of evidence is the frequent presence of Spirorbis even on those shells which 
have the periostracum preserved. It is only areasonable inference, too, to suppose 
that the shells of Carbonicola and Anthracomya, which nearly always occur with 
Naiadites, and under similar conditions as to the presence of Spirorbis, were not of 
a burrowing habit. 
Dr. John Young, of the Hunterian Museum, Glasgow, has made out that the 
genus Naiadites possesses, i common with the Aviculide, Pinnz, and others, an 
outer layer of prismatic cellular structure. He read a note” of this discovery 
before the Glasgow Geological Society on January 15th, 1880. He says, ‘So far as 
I have examined my Carboniferous Lamellibranchs, I find that this prismatic 
structure is confined to shells belonging to the Aviculide .. . and the Mytilide.” 
I figure on Pl. XVI, fig. 11, a portion of a shell of Naiadites from the Coal- 
measures of Knightswood, N.W. of Glasgow, which shows this structure. 
R. Ludwig had as long ago as 1863 pointed out the similar structure in a shell 
which he calls Anodonta obstipa (* Paleontographica,’ Bd. x, pl. iu, figs. 2h, f, 
x 100) from the left bank of the Uswa, near Nischni, Parogi, Perm. This shell 
has the appearance of an Anthracomya, in which genus, Dr. Young says, he has 
not been able as yet to demonstrate prismatic structure. 
1. Natapires mopronaris, Sowerby, 1836-40. Plate XVII, figs. 8—10, 12—30. 
AvicuLaA MopioLaRts, Sowerby. Trans. Geol. Soc., ser. 2, vol. v, pt. 3, pl. xxxix, 
fig. 18, read 1836, pub. 1840. 
No name. R. Garner. Nat. Hist. of County of Stafford, pl. 5, figs. 21, 22, 1844. 
AvIcULA MoprioLaRIs, Brown. Fossil Conch., p. 162, pl. lxi**, figs. 23, 24, 1849. 
Moptota FuNAtTA, Brown. Ibid., p. 172, pl. 1xxi, figs. 12, 18. 
“= suBrruNcATA, Brown. Ibid., p. 173, pl. xxii, figs. 15, 16. 

1 «Geol. Mag.,’ dee. 3, vol. x, 1894, p. 540. 
g p 
2 “Notes on some Carboniferous Lamellibranchs, their Mode of Occurrence and Observed 
Shell-structure,’” ‘Trans. Geol. Soc. Glas.,’ vol. vi, 1890, p. 223. 
