132 THE NAIADES OF MISSOtJRI 



ventrad in male, slightly swollen in female; whole shell medium 

 in size, small but more inflated; disk from beaks to central-ventral 

 edge sculptured with a row of a few large knobbed tubercles, 

 those of one valve alternating with the knobs of the other; post- 

 umbonal ridge with corrugations; epidermis yellowish-green to 

 dark brown. 



Internal Structures: — Cardinals upright, jagged; laterals 

 short, nearly straight at right angles to a rather broad interdentum 

 beak and branchial cavities moderately deep; nacre a pure, 

 stippled white. 



Diameter Locality 



35mni (Platte R., Platte R. Station) 



30mm (Marais des Cygnes, Athol) 



17mm (Platte R., Dixon Falls) 



12mm (Miss. R., Hannibal) 



9.5mm (Crows Fork, Fulton) 



The last two are measurements of juveniles of widely different 

 locality under far different ecological conditions, although the 

 shell characters are not very much different. The former shows 

 more of a rayed olivaceous epidermis and the latter a plain straw 

 color. The Mississippi juvenile, being more typical as in case of 

 most shells, is described here: — Shell sub-trigonal, valves inequi- 

 lateral with two knobs on one side and one on the other, darker 

 green epidermis below the knobs, rayed with interrupted V- 

 markings, beak sculpture irregular concentric undulations extend- 

 ing out on disk; nacre white, slightly tinged with pink. 



Miscellaneous Remarks: — Both as to structure of shell and 

 nutritive soft parts 0. reflexa is rather primitive, but as to marsupial 

 characters it naturally falls under the lower groups of the Lampsi- 

 linae. In North Missouri reflexa reaches a very large growth while 

 in Central Missouri it averages only about one-half the size; for 

 the two faunae this variation applies to many other species. Since 

 Drs. Lefevre and Curtis (19 12, pp. 137 and 138) have called 

 attention to the eccentric breeding habits and glochidial behavior 

 of reflexa the writer has followed up the breeding period rather 

 closely to find that it is gravid with early and late embryos, also 

 with glochidia, during June, July and August, but is sterile for 

 late Fall and mid- Winter, thus showing that this species has a 

 short period of gravidity, — a different reproductive habit from 



