7a DEVONIAN PAUNA. 



in having a larger umbo, in being much less transverse, in being more produced 

 and angular on the anterior side, and in its concentric ridges fading rather more 

 rapidly behind (so as to leave a rather larger smooth postero-superior portion), 

 and also on the front side of the median depression. 



Myacites striakdus, F. Romer,^ which is nearer in shape, has strong radiations. 



Allorisma plicatella, CEhlert,^ is very similar, except that it has no median 

 constriction, the concentric ribs being continuous over the whole back of the 

 shell. It thus shows the proximity of Leptodomus to Allorisma. 



Grammysia constricta, Hall,^ which its author first described as Grammysia 

 (Leptodomus?) constricta,'^ hut wiihont any rekrence to M'Coy's species, appears 

 to be a rather variable form, usually with considerably more numerous concentric 

 ridges, and of greater transverseness ; the American specimens figured by Hall 

 being normally twice as long as high, while ours, though perhaps variable, seem 

 to be only half as long again as the height. If this means that the species are 

 distinct, the name of the American form should become Leptodomus Hallil. 



Grammysia Hannibalensis, Shumard,^ r.grees more nearly with our shell in 

 dimensions, but its umbo is more anterior and oblique, and its constriction seems 

 to range more backward. 



Leptodomus Canadensis, Billings,® is closely allied, only differing in having a 

 much smaller umbo and in being longer. 



2. Leptodomus semisulcata, Soiverhy, sp. ? Plate IX, figs. 23, 24, 24 a. 



? 1839. MoDiOLA ? SEMISULCATA, Soioerhy. In Murchison's Sil. Syst., p. 617, 



pi. viii, fig. 6. 

 1841. Cypeicaedia semisulcata, Phillips. Pal. Foss., p. 36, pi. xvii, figs. 57 a — c. 



Description. — Shell moderate in size, very convex, transverse. Inferior margin 

 apparently slightly concave in front, gently convex behind. Postero-inferior 

 margin very convex. Postero-superior margin oblique. Valve with a vertical 

 concave constriction very near the anterior side ; and covered in with twenty or 

 thirty strong, elevated, regular undulations, which vanish rather suddenly before 

 reaching the posterior slope, and which are divaricated in the anterior constric- 



1 1844, F. Eomer, Ehein. Uebergangsgeb.,' p. 79, pi. ii, figs. 5, 5 a. 



2 1881, CEhlert, ' Mem, Soc. Geol. Fr.,' ser. 3, vol. ii, p. 34, pi. vi, figs. 3—3 b. 



3 1885, Hall, 'Pal. N. T.,' vol. v, pt. 1, No. 2, p. 377, pi. lix, figs. 4, 5?, 13—20, and pi. Ixxvii, 

 figs. 26, 27. 



* 1870, Hall, ' Prelim. Notice Lamellib.,' pt. 2, p. 58. 



5 1885, Hall, ' Pal. N. T.,' vol. v, pt. 1. No. 2, p. 381, pi. Ixi, figs. 29, 30, 33. 



8 1874, Billings, ' Palseozoic Fossils Canada,' vol. ii, pt. 1, p. 54, pi. v, fig. 1. 



