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 all impossible 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. The 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 

 Palasozoic 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 polymorplia 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. It is to be seen in the specimens figured on PI. XVII, fig. 15 ; 

 PI. 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 

 i<, of course, in the larger species a mechanical necessity that the hinge-plate 



