2 NAIADES OF MISSOURI 



morphologically and less liable to parallelism and convergence 

 We should not, however, lose cognizance of the importance con- 

 cerning shell characters. External factors may so shape the 

 shell, as to individualize it, but, under normal conditions, do not 

 often destroy all traces of indentification with some group. vSince 

 validity is so dependent upon the examination of both hard and 

 soft structures of large series of individuals, it is felt that the 

 species, as recorded for their extensive and intensive studies 

 herein, are fairly well settled within their genera. Fortunately 

 extensive field work has made this intensive study possible. On 

 one occasion the rare opportunity was given the author to make 

 a mussel survey of three hundred miles down the Osage River 

 in a row boat and ample time was given to the study of one of the 

 richest faunae of Missouri. Two papers, "Mussel vStudies Afield" 

 for the years 19 13 and 19 14, are now in MS. recorded matter of 

 which is submitted herewith. The ecologic facts secured in these 

 field surveys are especially interesting. For example, in the long 

 trip down the Osage, the largest tributary of the Missouri in the 

 interior of this state, it was found that the flat or compressed 

 forms found at the headwaters, where the water is shallower 

 and swifter, become more swollen and heavier shells further down 

 stream v.here the water is deeper and more sluggish. The most 

 valuable data gathered from these field excursions are the ecologic 

 as illustrated in Plates x — xiv. Much physiologic and mor- 

 phologic information, however, has been obtained from these 

 immediate studies in nature that could not be secured otherwise, 

 since actual observation have been made of the animal carrying 

 on the nutritive and reproductive functions there as well as 

 observed through aquarium studies (See Plate ix). More accurate 

 idea of the morphology (especially of soft parts as to color, form, 

 etc.) is obtained while the specimens are fresh and uncontracted as 

 examined afield. 



While this catalogued account is rather synoptical as to the 

 text of its animal and shell characters, yet it has been the effort 

 to be more complete than those literary works of somewhat 

 similar character that have devoted more space to the structures 

 of the shell; hence, much description will be found to be completely 

 given for the first time, notably those of M. heros (Say), 

 0. quadriila (Raf.), P. UUerhackii Frierson, 7?. tuberculata (Raf.), 

 U. tetralasma (Say), Las. suhorbiculata (Say), Prop, capax (Green), 



