140 THE NAIADES OF MISSOURI 



Internal Structures: — Cardinals rather stumpy and stout; 

 interdentum rather short and cut away; laterals very strong; 

 nacre white with stippled effect — a very valuable shell com- 

 mercially for this reason. 



Sex Length Height Diameter Locality 



(Marais des Cygnes, Rich Hill) 

 (Meramec R., Meramec Highlands) 

 (Osage R., Osceola) 

 (Gasconade R., Gascondy) 

 (St. Francis R., Greenville) 

 (St. Francis R., Greenvile). 



These last two measurements are those of two hyssijerous 

 juveniles identified by Dr. Howard and Prof. Clark. At first the 

 writer was inclined to call them L. luteola chiefly on the grounds 

 that both were found clinging by their byssi to a costata shell in 

 a bed where luteola predominated; however, this identification 

 was excluded on the basis of the presence of anterior rays and a 

 difference of umbonal sculpture being less prominent with the 

 ridges more broken in case of juvenile ligamentina as shown in 

 these two specimens. The byssus is attached to the upper posterior 

 part of the foot although it extends out antero-ventrad between 

 the valves. In the smaller juveniles the bysuss is about 120mm 

 long, in the larger 135mm. Both bear dense papillae on both bran- 

 chial and anal openings. Anterior end of outer gills lifted up very 

 high above the palpi, gills dark tan-color; branchial papillae rusty 

 red; epidermis yellowish with bright broad green rays — mostly 

 placed anteriorly. 



Miscellaneous Remarks: — While A^^. ligamentina may have 

 the widest general distribution of the North American Naiades, 

 yet it is not found anywhere in the interior oj this State north of the 

 Missouri River. However, it is the most common of species in 

 the Mississippi, the Des Moines and in all the interior drainage 

 south of the Missouri. It is not inclined to vary much from the 

 typical ligamentina of Lamarck; however, the Osage River con- 

 tains some forms that are somewhat puzzling due to ecological 

 conditions that erode the epidermis and distort the shell of this 

 species (and of other characteristically rayed species, for that mat- 

 ter) , but these are of rare occurrence in the Osage from the center of 

 its course to its mouth. As mentioned elsewhere this local effect 

 may be traced to the chemical reaction of the mineral water of 

 the springs' region. There are no species it may be confused with 



