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, Authracoptera tumida at p. 82 of the ' Memoirs of the Geological Survey of 

 Scotland,' Explanation of Sheet 31, where he says : "J. tumida 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. tumida 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." Ou carefully comparing both 

 descriptions, there is no real point of distinction offered, and I cannot but think 

 that both A. obesa, and A. tumida 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. I figure three of the most 

 perfect, PI. 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 

 geuus Naiadites, with the result that I found distinct traces of it in most species. 



7. Naiadites crassa (Fleming). Plate XX, figs. 1 — 11. 



Modiolus, sp., Fleming. Ediu. Phil. Journ., vol. vii, p. 246, pi. ix, fig. 3, 1S25. 

 Mytilus crassus, Fleming. British Animals, p. 412, 1828. 

 Modiola, sp., Bhind. Age of the Earth, p. 167, pi. ii, fig. <j. 



Ayicula modioliforme (sic), Brown. Foss. Conch., p. 162, pi. kvi*, fig. 19, 1819. 

 Mytilus (myalina) crassus, King. Mon. Perm. Foss. Eug., Pakeontograph. Soc, 



p. 159 (uo description), 1S50. 



— crassus, Morris. Gat. Brit. Foss., 2nd edit., p. 214, 1854. 

 Avicula MODiOLiFORiiis, Morris. Ibid., 2nd edit., p. 162, 1854. 



— — Salter. Memoirs Geol. Surv. Scotland, No. 32, p. 146, 



1861. 

 Mtalina cbassa, Huxley and Etheridge. Cat. Foss. Mus. Pract. Geol., p. 110, 



1865. 



