NAIADES OF MISSOURI 45 



or less parallel to post-ridges; beaks rather low and full, sculptured 

 with numerous corrugated or double-looped ridges extending out 

 as prominent nodules on the post-umbonal-looped ridges and 

 zigzag, or WM-shaped, ridges on the umbonal region and upper 

 part of disk; epidermis black, more or less dull color. 



Internal Structures: — -Cardinals moderately heavy, double 

 in both valves; laterals very long, not much curved, interdentum 

 short, narrow; beak and branchial cavities rather deep; muscle 

 scar — expecially the progressive impression and that of anterior 

 retractor — very deep ; nacre white (often with rusty spots) varying 

 to pink. 



Sex Length Height Diameter Locality 



9 150 X 116 X 56mm (Platte. R, Dixon Falls.) 

 c? 153 X 117 X 56 " ( " " Garretsburg.) > 



39 " (Osage R., Warsaw) 



25 " ( " " Monegaw Springs) 



i2.5mni ( " " Warsaw) 

 9.5 " ( " " Proctor) 



The young and juvenile shells of the last two respec- 

 tive measurements are very profusely sculptured — no part 

 of the external surface being smooth — yet no undulations appear 

 as seen in adult. Beaks low, corrugated; slopes of post-dorsal 

 ridge finely costated; post-ridge with coarse, apiculated or spurred 

 nodules; center and anterior of disk covered with irregularly 

 placed V-shaped ridges and with scattered tubercles; valves 

 extremely flat; nacre sky blue, irridescent. "Work up old shells 

 from the young ones" is Mr. Walker's advice. 



Miscellaneous Remarks: — This most ponderous shell of 

 the Naiades is typical in North Missouri and the Mississippi, 

 but the typically massive shell is not found in Central Missouri, 

 and, so far, it is not reported at all for the southern slope of the 

 Ozarks in this state. As a rule, heros is only found in the large 

 rivers; however, in this State the type is found in the Missouri 

 Platte — a rather small river, while the medium sized one is found 

 in the largest river of the interior — the Osage — where it assumes 

 the small form perhaps approaching that of dombeyana Valen- 

 ciennes. "Giant heros,'' as this species is often called, is most 

 frequently found in the deepest depressions of mud bottom with 

 a substratum of solid limestone. It hardly ever moves from these 

 situations and perhaps because of this inactivity it accumulates 

 its heavy shell. Because of the following peculiar characters the 



