310 THE PEARLY FRESH-WATER MUSSELS— SIMPSON. voi.. xvm. 



between the two genera. The binge is either destitute of teetli or 

 exhibits thein only in a rudimentary condition, and the nacre is less 

 brilliant, as a rule, than it is in the TJnios. Anodonta implicata, Say, 

 and A. fenotilli, Heude, are greatly thickened usually in the anterior 

 region, often becoming as solid as some of the heavier TJnios. A. angu- 

 lata, Lea, is also quite a solid sliell, and is generally strongly inflated 

 and sharply angled on the posterior slope. According to Hemphill.' it 

 was found in hard, clayey gravel, in the Snake River, burrowing so that 

 only the solid, angled posterior end came to a level with the surface. 

 This is no doubt a modification of the shell in order to enable it to 

 resist the shock of the currents, as specimens of the same species taken 

 from still AA^aters are thinner, more comi)ressed, and almost entirely 

 destitute of the posterior angle. This species has usually rudimentary 

 teeth, and in the young both cardinals and laterals are often perfect. 

 Tlie shell is incurved in front of the beaks and it may be a true Unio. 



The animal of Anodonta is essentially the same as that of Unio, and 

 there can be but little doubt that the two genera are very closely 

 related. Whether Anodonta or Unio is the older it is impossible in the 

 present state of our knowledge to tell, as it is quite probable that some 

 of the more ancient forms referred to the former genus are not Auodon- 

 tas at all. There can be, I think, little doubt that the thick shells and 

 hinge teeth of the Unios were developed in order to enable them to live 

 in currents, as they are generally inhabitants of streams; while the 

 thin, edentulous shell of Anodonta is caused by its living in still water; 

 the genus belonging, for the most part, to ponds, lakes, and canals. 



The distribution of the true Anodontas is confined to the northern 

 hemisphere — for the most part north of the Tropic of Cancer, the 

 so-called species of South America being Glabaris, and those of trop- 

 ical Africa belonging to Sjyatha and Mutela, all genera of the family 

 Mutelidie. The Anodontas are found throughout Xorth America as far 

 south as southern Mexico ; in northeastern Asia; in Japan and China, and 

 in the great region north and west of the Himalayas; also throughout 

 all Europe and northern Africa to the Desert of Sahara, excepting in 

 the Nile, which is peopled with Spathas and Mutelas. The embryo is a 

 glochidium, and probably attaches itself to fishes as does that of Unio. 

 It is very difticult to draw the line between the genera Unio and Ano- 

 donta. In the United States there occurs a small group of species, some 

 of which have been placed with Margaritana, such as M. elliotti, Lea, M. 

 tombigheensis, Lea, and M. elliptiea, Lea; and others with Anodonta, 

 such as A. edentnla, Say. These species are, with some others, closely 

 related by characters of the shell and soft parts, and all undoubtedly 

 belong to a single group of one genus. In many cases in this group, 

 even in A. edentula, there are fairly developed cardinals and even rudi- 

 mentary laterals, and this, Avith the general character of tlie shells, 

 leads me to place the species in Unio. Anodonta ferussaciana, Lea, and 



i Zoe, I, No. II, p. 326. 



