46 IOWA ACADEMY OF SCIENCES. 



tribution of the various forms Avas adduced as an argument 

 in support of identity particularly of those species which 

 occur in Texas and in Georoia. 



The type of the group is the very small form descril)ed by 

 Barnes from the waters of Ohio. This form with consider- 

 able variations occurs throughout the eastern half of the 

 Unites States as far as the State of New York. To the south 

 it ranges westward to Texas and has there been subjected to 

 such environmental conditions as to become very much pro- 

 longed posteriorly. The sexual variations here are so marked 

 that the two forms, male and female, have been described 

 under at least two specific names. In Georgia occur two or 

 three forms which have been called species mainly on the 

 difi'erences of color of the nacre. Some of these at least are 

 but modifications of the common Unio parvus of the North. 



The group includes the following species descril)ed by Dr. 

 Isaac Lea, of which list those in italics are believed to be syn- 

 onyms. In the list the forms Avhi'ch it is proposed to recognize 

 as species ai-e given in the order of the date of their descrip- 

 tion, so that, if for any reason the reader should M'ish to de- 

 termine the chronologic arrangement of the species it will 

 be possible to do so. 



Unio glans Lea. 



(Read May 7, 1830.) 



"Observations on the Genus Unio,'' Vol. I, p. 92, PI. VIII, 

 Fig. 12. 



Unio pauhis Lea. 



(.Read October 2, 1840.) 



"Observations on the Ge^nis Unij,'' Vol. Ill, p. 51, PI. XV, 

 Fig. 29. 



Unio minor Lea. 



(Read August iS, 1843.) 



"Observations on the Genus Unio," Vol. IV, p. 27(3, PI. 

 XXXIX, Fig, 3. 



