64 OBSERVATIONS ON NAIADES, 



consequence of which is, that when the horn is sufficiently elevated, 

 the line of the opening at the hase of the shell has diverged from the 

 plane of the valves into an obtuse angle at this point. The deposit 

 of the secretion is then reversed, and the line of the opening at 

 the base is soon restored to the plane of the valves. If another 

 horn is to be formed, this lateral increase of the edges is carried on 

 until the same effect is produced on the other side of the shell. The 

 natural consequence of this alternation is a depression on the outer 

 side of one valve corresponding to the horn on the other, and thus we 

 ever find it. In one of my specimens the turn is so short, after the 

 formation of the last horn, that this side passes over the other and forms 

 a plane one third of an inch, so that the specimen presents the curious 

 phenomenon of a shell standing erect on its base, when placed on a 

 smooth surface. 



The plicae or folds are formed on the same principle. In the basal 

 and posterior margins of the plicated species we may see the line of 

 opening undulated by every fold, and when the deviation from the 

 plane takes place in one valve it is followed up always by the other. 

 In the Unio tuherculatus, when tuberculated to the edge, these tuber- 

 cles cause it to be crenate. 



In the Unio metanever we are presented with different elevations on 

 the disks. This beautiful and interesting species is furnished with 

 elevations, small at the beaks, and enlarging towards the base along 

 the umbonial slope. These elevations are antagonist, and being 

 formed at the same time, we consequently see, in a certain stage of 

 growth, quite a knob at the angle of the basal margin. In the Unio 

 lacrymosiis, which is furnished with smaller elevations on the umbonial 

 slope, we find them to alternate. 



When making some observations on " colour" in my last memoir, I 

 mentioned that "the green irregular spots and marks" on the interior 

 of the valves were "accidental, perhaps the effect of disease." Subse- 

 quent observation led me into an examination of these marks, and the 

 result is a perfect conviction of their being epidermal matter, evidently 

 placed occasionally between successive layers of nacre, as it were in 

 anticipation of a future erosion of the beaks. In a specimen of Unio 

 ptistulosus (herein described) in my cabinet, erosion has taken place 



