THE NAUTILUS. 19 



Lam., which at that time he had been unable to identify from the 

 original description. In his description of Unio solidus (Obs. II, p. 

 13), read Dec. 19, 1834, he compares that species with undatus and 

 distinguishes it by " being more rounded at the basal margin, by its 

 more elevated beaks and by its color." 



In his description of Unio planus (Obs. Ill, p. 51), read Oct. 2, 

 1840, he differentiates that species from both obliquus and trigonus 

 as being more rounded at the base, the emargination being very small 

 and being higher in the beaks than trigonus and more flattened there 

 than obliquus. 



In the same paper he remarks (p. 54) that his dolabelloides stands 

 between undatus and cor. 



It is curious to note that as late as Feb. 19, 1841, he seems still 

 to ignore obliquus on occasion and to use undatus (Obs. Ill, p. 69). 



But after that date, undatus is not referred to except in his Recti- 

 fication and Synopses. 



In his description of Unio tumescens (Obs. iv, p. 45), read May 2, 

 1845, he says that that species is allied to trigonus, but differs in be- 

 ing more rounded and in having rays. The comparison is certainly 

 not a very apt one to say the least. 



In his description of Unio chunii (Obs. ix, p. 18), read June 3, 

 1862, he states that this species is closely allied to trigonus, but may 

 be distinguished by being more lenticular in form and in not having 

 so sharp an umbonial slope. The undulations of the beaks of that 

 species, however, are few, as in trigonus, and follow down the angle 

 of the umbonial slope. In the same paper (p. 21) in his description 

 of Unio riddellii, he remarks that that species belongs to the group 

 of which trigonus may be considered the type, but differs in being 

 rounded, even more inflated and in the character of the undulations, 

 which are " remarkably close," while in trigonus they are few and 

 follow down the angle of the umbonial slope for a short distance." 



It is evident from these comparative remarks that, in Lea's mind, 

 trigonus was a shell with prominent beaks, though less so than in 

 solidus and plenus, with a rather wide basal emargination, a sharp 

 umbonial angle, and having the beak's undulations few and following 

 down the umbonial angle for a short distance. 



These specifications apply accurately to the shell above identified 

 as undatus Barnes, and do not apply to any other Quadrula of the 

 Ohio drainage. 



