212 



BRITISH MOLLUSKS. 



American species of Unio have been observed to spin a byssus in 

 cases of emergency. In South America M. D'Orbigny collected, 

 at the Rio Parana, above Corrientes, an Anodon attached by a 

 byssus ; his genus Byssanodonta will not however hold, if depen- 

 dence cannot be placed on this function as a generic character. 



About six hundred and thirty species of Naiads, with along array 

 of synonyms, are now admitted, reckoning only eleven as belong- 

 ing to Europe, forty-three to Asia, seventeen to Africa, and three 

 to Australia. Of the remaining species, seventy-six belong to 

 South America, and four hundred and eighty to North America. 

 There are seventy-four species of four genera in the Eastern Hemi- 

 sphere, and five hundred and fifty-six of six genera, in the Western, 

 distributed as follows : — ■ 



Genera. 



P. 

 o 



U 



3 



.2 

 ■3 



r3 

 < 



03 



b 



■A 



s 



Unio ....... 



10 



33 



6 



2 



430 



28 



Anodonta . . 



1 



8 



2 



1 



50 



32 



Dipsas 



— 



2 



— 



— 



— 



— 



Iridina 



— 



— 



5 



— 



— 



— 



Spatha 



— 



— 



4 



— 



— 



— 



Syria 













3 



Castalia ..... 



— 



— 



— 



— 



— 



2 



Monocondylcea . 













8 



Mycetopus 













3 



The genera Unio and Anodonta are mostly associated together, 

 the first being much the more plentiful in species, excepting in 

 South America, where the toothless Naiads predominate over 

 the toothed. Of the eleven European species, we have four in 

 Britain, two confined to England, two diffused throughout the three 

 kingdoms. 



They are of the genera — 



1. Anodonta. Shell thin and toothless. 



2. Unio. Shell rather stout, hinge composed of interlocking 



erect teeth on the anterior side, and elongated marginal 

 teeth, which are sometimes obsolete, on the posterior. 



