122 THE AMERICAN MIDLAND NATURALIST 
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 STRUCTURES:—Shell 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 STRUCTURE:—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 
O.f92 TE 73) CR ome (Osage R., Schell City, Mo.) 
ON Ss Geax Sine ee sonny (Gasconade R., Gascondy, Mo.) 
ron enh Stan fume, hme eas Ut (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:—Amblema costata Rafinesque is 
without question the Unio undulatus Barnes, but the trinomial 
name, Amb. 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 ‘‘Amblema.’’ ‘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 Nazad species in this 
State south of the Missouri River. Most of the students of Nazades 
have returned the results of their studies of the Missouri: Amblemae 
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 
oe 
