30 UNIONID^. 



tion, there are no lateral teeth in Mytilidce, and in Mij- 

 tilus the cardinal teeth are conspicuous. In the Unio- 

 nidcB, on the other hand, the lateral teeth are always, 

 and the cardinal teeth never, present. I am quite aware 

 that this last statement will be objected to by all those 

 conchologists who believe that, at all events, the shells of 

 Unio are furnished with cardinal teeth. But I venture 

 to submit that these teeth are lateral, and not cardinal ; 

 that they are not, like the cardinal teeth in the SphcB- 

 riicke, placed at a right angle to the hinge-line, but that 

 they are, on the contrary, parallel to it ; and that they 

 are always lamellar and form more or less elevated ridges, 

 like the true lateral teeth in other bivalves. In the 

 genus Anodonta, indeed, the lateral teeth are not so 

 strongly developed as in Unio, and they may in some 

 cases be considered as rudimentary ; but in nearly all 

 the species of Anodonta these teeth form a well-defined 

 and often sharp crest, especially on the posterior or liga- 

 mental side. The unusually great length and strength 

 of the ligament in Anodonta seems to render the use of 

 lateral teeth in supporting the hinge almost unnecessary ; 

 and in this, as well as in many other cases of a similar 

 kind, the original form of such organs is retained in an 

 imperfect state, although their use has ceased to exist. 



The study of the European members of this family 

 has for a long time attracted the attention of continental 

 naturalists; and Carl Pfeiffer, Rossmassler, and Henri 

 Drouet have especially applied themselves to this diffi- 

 cult task. A valuable monograph has been published 

 by the last-named naturalist, entitled " Etudes sur les 

 Naiades de la France ;^^ the work being dedicated to the 

 late King of Portugal, whose devotion to conchology 

 was the more remarkable because this branch of natural 

 history has not been cultivated by many crowned heads. 



