NAIADITES CRASSA. 147 
and are evidently younger forms. Mr. Etheridge says: ‘“‘ The much more central 
position of the diagonal ridge, greater convexity of the shell, and the sigmoidal 
margin of the posterior end ’’—the latter character, however, is not shown in the 
large type-specimen,—“ at once distinguish A, obesa from” A. carinata, A. quad- 
rata, and A. modiolaris. These shells, however, all at times have a sigmoidal 
posterior margin, and A. carinata is often very much swollen ; but I regard the 
more central position of the diagonal ridge, and its obesity, as characteristic features. 
It is just these points that Mr. Etheridge also advances as diagnostic of his other 
species, Anthracoptera tumida at p. 82 of the ‘ Memoirs of the Geological Survey of 
Scotland,’ Explanation of Sheet 31, where he says: ‘‘A. twmida has for its essential 
characters the great tumidity of the valves, strong and nearly median diagonal 
ridge, and prominent beaks.’’ He states, however, further on that ‘‘ the general 
outline of A. twmida approaches much nearer to another species of the same genus 
lately described by me, A.? obesa, but many of the points already indicated will 
also serve as distinctive ones in this case.’ On carefully comparing both 
descriptions, there is no real point of distinction offered, and I cannot but think 
that both A. obesa and A. twmida belong to the same species, which therefore 
retains the name of obesa. The shells from Boness in the Survey Collection are 
all more or less crushed and pressed out of shape. 1 figure three of the most 
perfect, Pl. XIX, figs. 12—14. At page 80 I pointed out that I suspected the 
identity of the shell from the Kilrenny Mill beds with Mr. R. Etheridge’s 
A. ? obesa, but at that time I had not satisfied myself that an anterior hinge-tooth 
was present in the genus Naiadites; in fact it was the presence of a well-marked 
tooth in the form that made me search for traces of it in the other species of the 
genus Naiadites, with the result that I found distinct traces of it in most species. 
7. Narapites crassa (Fleming). Plate XX, figs. 1—11. 
Moptouvs, sp., Fleming. Edin. Phil. Journ., vol. vii, p. 246, pl. ix, fig. 3, 1825. 
Myrinus crassus, Fleming. British Animals, p. 412, 1828. 
Moprona, sp., Rhind. Age of the Earth, p. 167, pl. ii, fig. g. 
AVICULA MODIOLIFORME (sic), Brown. Foss. Conch., p. 162, pl. lxvi*, fig. 19, 1849. 
Myritus (Myauina) crassus, King. Mon. Perm. Foss. Eng., Paleontograph. Soc., 
p- 159 (no description), 1850. 
— crassus, Morris. Cat. Brit. Foss., 2nd edit., p. 214, 1854. 
AvicuLa MopIoLIFoRMIs, Morris. Ibid., 2nd edit., p. 162, 1854. 
== — Salter. Memoirs Geol. Surv. Scotland, No. 32, p. 146, 
1861. 
Myatina crassa, Hurley and Etheridge. Cat. Foss. Mus. Pract. Geol., p. 110, 
1865. 
