342 THE PEARLY FRESH-WATER MUSSELS— SIMPSON. vol.xviii. 



sphere. The presence of a species in what are probably Triassic strata 

 in Texas, with stronglj^ radial beak sculpture, a character nowconftued 

 to the I'niouids of the two areas in question, is evidence in this direc- 

 tion . The forms with variously sculptured beaks which bear the embryos 

 in the outer gills may be a more recent, vigorous stock, and it is pos- 

 sible that they have taken possession of the lakes aud streams of the 

 northern hemisphere and driven these older types to the southward. 



The same thing may be true with the Mutelidie, whose northernmost 

 limit in the Old World is the lower Nile, aud in the New, southern Mex- 

 ico. And if the Cretaceous fossil now known as Spatlia (/alloprorin- 

 cialis, Matheron, from the mouths of the Rhone, is reallj' a member of 

 that geims, it would give color to this tlieory, which necessitates no vio- 

 lent changes of land and sea to account for present Naiad distribution. 



To briefly sum up: The old arrangement of the families Mutelidie 

 and TJnionidai based uj)on the x>resence of siphons in the former and 

 their absence in the latter can not stand, as this character may be 

 developed or wanting in a single genus or even species. Ihering's 

 redefinition of the families, in which the former is founded on the fact 

 that the embryo is a three-parted lasidium, and that of the latter a 

 glocliifJium, with the animal inclosed in a bivalve shell, agrees essen- 

 tially with the characters of the hinge and shell generally. Those 

 forms which would seem to belong to the Mutelida3 have irregularly 

 taxodont teeth or vestiges of them, while the Uuiouidte have schizodont 

 teeth, which are arranged as cardinals or laterals, or both, though they 

 may be merely rudimentary or even sometimes absent. The Naiades 

 seem to be capable of being grouped into assemblages of related forms 

 which have a more immediate common ancestry, and on the basis of 

 this grouping we find them distributed into eight provinces, four of 

 which are in the Old World aud essentially agree with tlie regions of 

 animal life of Wallace and Sclater. 



These may be tabulated as follows: 



AREAS OF THE NAIAJ) RECJIONS.' 



("Europe. 

 „ , . J Northern and western Asia, 



raiearctic < ^^^^^ Africa to the Desert. 



[Pacilic drainage of North America. 



Ethiopian Africa sonth of the Sahara. 



^ . . , { Asia south of the Himalayas. 



^^^®"^'^^ } East Indies to the Solomon Islands. 



'' 'Australia, 



Australian \ Tasmania. 



( New Zealand. 

 Neotropical South America. 



( Central America. 

 Centra] American-? Mexico east of the Cordillera. 



( Cuha. 



r Entire Mississippi Valley and the Gulf drainage from west Florida 



I to the Eio Grande. 



Mississippian { Mackenzie River system. 



Red River of the North. 



[ Great Lakes. 

 . ,, .. { Lower St. Lawrence and rivers of eastern Canada. 



-'^^^^""•^ \ Atlantic drainage of the United States. 



'For map of Naiad Regions see Plate IX. 



