130 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
because they do not occur with a typically marine fauna as a rule,—the 
one possible exception being the Naiadites crassa of the Beith and Pitlessie 
beds, and I am in doubt as to the real conditions under which these deposits were 
laid down. 
At Pitlessie the shells are accompanied by Stigmaria and other vegetable 
remains, scales of Megalichthys and Spirorbis, together with Aviculopecten, 
Encrinites, and corals. The shells are in a much disturbed state, and very few are 
whole or possess the valves in contact ; and with such a mixed fauna and flora, the 
bed may well have been the result of a wash; but, as I have remarked, very many 
shells exhibit fractures repaired during life. It is not at allimpossible that Naiadites 
crassa was of marine habitat, and that the dwarfing of the other members of the genus 
may be due to a change of condition of environment. At Beith, the other habitat 
of this shell, it is found in a bed resting on coal associated with fish remains. Here 
the shell is in a somewhat better preserved state, the valves often being in 
contact; the evidence shows that at this place the shell was truly marine; 
certainly its extremely local occurrence is very marked. The three pit-like 
muscular scars within the anterior part of the shell are characteristic of the 
genus. It is difficult to be accurately certain as to the several muscles which each 
represents. ‘he anterior two pits are much larger than the posterior, and 
possibly represent a bifid anterior adductor, while the posterior pit represents 
the attachment of the long anterior byssal muscular bundle. M‘Coy (‘ British 
Paleozoic Fossils,’ p. 492), speaking of Myalina, says, “The small impression 
over the large anterior adductor I find (in the recent Mytili with rostral plates) 
is the scar of the insertion of the adductor from the opposite valve, instead of 
retractors of the foot, as commonly supposed, the larger impression being the 
scar of origin thereof.” My own dissections of Dreissenia polymorpha hardly bear 
out this, but Professor M‘Coy may be referring to Mytilus bilocularis (Septifer, 
Recluz), with which I am not acquainted. 
Mr. Salter mentions the fact of this genus, Anthracoptera, possessing in front 
an obscure tooth in the hinge; it is curious, if he had good enough specimens to 
ascertain this fact, that he did not notice the striated hinge-plate. The tooth is 
very obscure in the smaller species, but even in these it is to be made out in well- 
preserved examples. Itis to be seen in the specimens figured on Pl. XVII, fig. 15; 
Pl. XIX, figs. 1—7. 
The striated hinge-plates seem to have been in contact in the smaller forms 
to such an extent that nothing whatever of them was visible when the valves were 
together, but in the larger shells of Naiadites crassa this hinge-plate is at such an 
angle that the valves could only have been in contact along the lower border, 
leaving an elongate pit, triangular in section, which lodged the cartilage; and it 
is, of course, in the larger species a mechanical necessity that the hinge-plate 
