22 THE NAUTILUS. 



negative. It follows therefore that trigonus is identical with unda- 

 tus or is a distinct species. 



In considering this question, it is to be borne in mind that nearly 

 all of our Unionidce are subject to a large amount of variation and 

 that the wider the range of a species, the greater the amount of 

 variation it exhibits under the influences of diverse conditions of 

 local environment. 



Trigonus (using the term in its broad, current acceptation and not 

 confining it to the form of Lea's type) has a very extensive range. 



In the Mississippi Valley from Minnesota (Grant) south to Ark- 

 ansas (Call) and northern Louisiana (Frierson); in the Ohio drain- 

 age it ranges east through Ohio (Sterki) but apparently does not ex- 

 tend into Pennsylvania (Ortmann) nor southwestern N. Y. (Mar- 

 shall); through the ancient post-glacial connections of Lake Michigan 

 with the Mississippi and Lake Erie with the Ohio, it has invaded 

 the St. Lawrence system and is found in the lake drainage of Wis- 

 consin, Illinois and southern Michigan, whence it ranges east as far 

 as Buffalo, N. Y. (Marshall) and Port Dover, Ont. (Whiteaves), 

 but does not appear in the valley of the Ottawa (Latchford); in Ken- 

 tucky it has been recorded from the Barren River (Walker) and 

 Warren Co. (Price) in the Ohio drainage; apparently it does not 

 occur in the Tennessee drainage area at all; but, curiously enough 

 it reappears in the Alabama system where it is usually, but erron- 

 eously, called chunii Lea. 



Through Texas, southern Louisiana and Mississippi, the trigona 

 group is represented by a number of " species," whose relations with 

 each other and with trigona are uncertain, and can only be defi- 

 nitely determined by a much larger amount of material than is ap- 

 parently accessible at the present time. Trigona as such, so far as 

 I have been able to ascertain, has not been listed from any of those 

 States except northern Louisiana. 



While trigona, commonly so-called, throughout this enormous ex- 

 tent of territory sustains its specific identity sufficiently to be recog- 

 nized in most cases without difficulty, yet, as might be expected, it 

 exhibits in different parts of its range a considerable amount of vari- 

 ation. 



Thus in the Mississippi, Illinois and Fox rivers, the high trian- 

 gular form, which I have identified with undatus, is the prevalent 

 phase; in the Ohio a more equilateral form (typical trigona) occurs 



