CHAP. X. UNIONID^ PRIMARY CIRCLE. 263 



ally, however, the shell becomes stronger, more elon- 

 gated, and the hinge margin again thickened : this is 

 particularly observable in a species from the Nile *, 

 placed by some writers as an anodon, and by others as 

 an Iridina ; but to which it really belongs is not, at 

 present, material. Suffice it, that thus the series of the 

 Unionidce returns into itself, and forms as perfect a 

 circle as any, perhaps, contained in this volume. 



(248.) The proportion which the three aberrant di- 

 visions bear to the pre-eminently typical, is nearly the 

 same in this as in all natural groups of the animal 

 kingdom. By far the most numerous in forms and 

 species is the typical genus Unio, which contains, in 

 fact, more than all the rest put together ; the fewest, 

 perhaps, are in Hyria and Iridina : hence it naturally 

 results, that the interval between the species, in these 

 two latter, are wider than in the former. But these little 

 intervals, more or less, occur in every group in nature ; 

 and provided that no animals are found in other groups, 

 which appear to fill up these chasms, we may be quite 

 sure our series is correct. In the Unionidce, this per- 

 plexity can never happen ; their general structure is so 

 peculiar, that no intervals among them can be filled up 

 by shell-fish of other groups : hence we may feel per- 

 fectly sure that the break in the line of continuity ori- 

 ginates either from a few links having not yet been 

 discovered, or that they exist only in a fossil state. 

 The proximity, for instance, between Iridina and Ano- 

 don is so close, that there is not the slightest link 

 wanting ; but between Iridina and Hyria the difference 

 is more marked. Are we, then, to conclude from this, 

 that these two are not also connected ? Certainly not. 

 We know that Hyria is imperceptibly united to Unio, 

 on one side ; and if, on the other, it approaches nearer 

 to Iridina than does any other genus, we have induc- 

 tive proof that they follow each other, although one or 

 two of the links may appear wanting. 



This may possibly be the A. rubens Lam. 



s 4 



