40 NAIADES OF MISSOURI 



are similar, except slightly larger (0.210 x 0.220 mm). Like other 

 members of this genus there is no trace of brilliant colors of the 

 soft parts such as red or orange as seen in the Fusconaia or 

 Alasmidonta. 



SHELL CHARACTERS. 



External vStructurEs: — vShell elliptically subquadrate, com- 

 pressed; rather thick and heavy umbones, not elevated, slighly 

 inflated, sculptured by five, coarse concentric ridges most pro- 

 nounced at base of post-umbonal ridge, slighlty alated anteriorly; 

 dorsal ridge high, with four or five upcurved costae; posterior 

 half of shell crossed with five or six oblique undulations with 

 shallow valleys; epidermis reddish brown to yellowish. 



Internal vStructure: — Cardinals very heavy, double in 

 both valves; laterals heavy, serrated; interdentum broad, thick; 

 beak cavities deep narrow, crevice-like; muscle scars well impressed, 

 vein markings on extra-antero pallial border; nacre white, often 

 rusty spotted, with blue irridescence at posterior end. 



Sex Length Height Diameter Locality 



cf 112 X 73 X 36mm (Osage R., Schell City, Mo.) 



? 76 X 55 X 32 " (Gasconade R., Gascondy, Mo.) 



d^ 32 X 25 X 15 " (Chariton R., Kern, Mo.) 



Juvenile shells have coarse concentric undulations upon the 

 umbonal region — especially at base of the post-ridge where they 

 are upcurved; a single broad undulation at the post-ventral 

 position of shell, a slight alation just anterior to the lunule; color 

 of epidermis olivaceous. 



Miscellaneous Remarks: — Amhlema costata Rafinesque is 

 without question the Unto undulatus Barnes, but the trinomial 

 name, Amh. plicata costata (Raf.), is used for taxonomic reasons 

 as has already been explained under the remarks on this genus 

 concerning the nomenclature incident to the revival of Rafin- 

 esque's ''Amhlema.''' This species is very seldom seen in typical 

 form in North Missouri, (never in North-West Missouri) and for 

 that matter, it is also scarce in Central or South Missouri — but 

 its actual forms are most abundant of all the Naiad species in this 

 vState south of the Missouri River. Most of the students of Naiades 

 have returned the results of their studies of the Missouri Amhlemac 

 indicating a greater prevalence of the " undulata" rather than the 

 "plicata" form — especially for the swift clear- water mountain 

 streams of the South. By actual surveys of some streams of 



