SCAMMON : THE UNIONID.K OF KANSAS, PART I. 281 



tion with the descriptions, it may be well to define them. 

 The "longitudinal axis" of the shell is considered to be a 

 line drawn through the ventral margins of the large anterior 

 and posterior muscle cicatrices. The longest line which can 

 be passed through the valve parallel to this axis is given as 

 the length of the shell. The height is considered to be the 

 length of the line passing through the umbones of the shell 

 at right angles to the longitudinal axis. By taking this con- 

 ception of the height, the sex variation of the shells of the 

 higher Unios is eliminated from this measurement. It has 

 seemed best to give the position of the umbones in relation to 

 the length of the shell in more definite terms. To secure 

 this relation in numerical terms, a line is dropped perpendicu- 

 larly from the tip of the umbones to the greatest longitudinal 

 axis, and the distance from the intersection of the two lines 

 to the anterior margin of the valve is given as a decimal frac- 

 tion of the entire length of the greatest longitudinal axis. 

 This relation has been termed the umboidal ratio. 



The two following terms have been employed in the de- 

 scriptions, and are suggested for general use. First, the 

 word "interdentum," as signifying the plate bridging the 

 space between the pseudocardinals and the laterals ; sec- 

 ond, the term "branchial outline," as indicating the slight 

 groove to be seen in the cavity of many of the more solid 

 Unios, and which is caused by the slightly raised outline of 

 the branchiae. 



The statements concerning the general distribution of each 

 species are derived in the main from Simpson's "Synopsis," 

 and this information is therefore in almost every case to be 

 credited to that author. In considering the distribution 

 within the state, an effort has been made to extend due 

 credit in all cases where exact localities are named. In deal- 

 ing with this local distribution, Doctor Call's work (4) has 

 been most valuable. 



I wish here to express my thanks to those who have aided 

 me in this work. These are due particularly to Dr. C. E. 

 McClung, under whose direction this work was done, and 

 who has by his numerous kindnesses and suggestions ren- 

 dered it possible. The following gentlemen have kindly per- 



