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, 1 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 a reasonable 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, in common with the Aviculida3, Pinna?, and others, an 

 outer layer of prismatic cellular structure. He read a note 2 of this discovery 

 before the Glasgow Geological Society on January 13th, 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 Aviculidas . . . and the Mytilida3." 



I figure on PI. XVII, fig. 11, a portion of a shell of Naiadites from the Coal- 

 measures of Knightswood, N.W. of Glasgow, which shows this structure. 



R. Ludwiff had as lone; aefo as 1863 pointed out the similar structure in a shell 

 which he calls Anodonta obstipa (' Palgeontographica,' Bd. x, pi. iii, figs. 2 h, f, 

 XlOO) from the left bank of the Uswa, near Nisclmi, 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 structm^e. 



1. Naiadites modiolakis, Soiverby, 1836-40. Plate XVII, figs. 8—10, 12— 30. 



Avicula modiolaeis, Sowerhy. Trans. Geo]. Soc, ser. 2, vol. v, pt. 3, pi. xxxix, 



fig. 18, read 1836, pub. 1840. 

 No name. B. Garner. Nat. Hist, of County of Stafford, pi. e, figs. 21, 22, 1844. 

 Avicula modiolaris, Brown. Fossil Conch., p. 1G2, pi. lxi **, figs. 23, 24, 1849. 

 Modtola funata, Bvoion. Ibid., p. 172, pi. lxxi, figs. 12, 13. 



— subtruncata, Brown. Ibid., p. 173, pi. lxxii, figs. 15, 16. 



1 ' Geol. Mag.,' dec. 3, vol. x, 1894, p. 540. 



2 "Notes on some Carboniferous Lamellibranchs, their Mode of Occurrence and Observed 

 Shell-structure," 'Trans. Geol. Soc. Glas.,' vol. vi, 1890, p. 223. 



