THE NAIADES OF MISSOURI 97 



specimens of this form to find it with the same breeding season 

 as its parent species. 



II — Sub -Family Anodontinae Ortmann. 



1911a — Anodontinae Ortmann, An. Car. Mus., IV, p. 336; 1912b, 

 An. Car. Mus., VIII, pp. 278-300. 



Animal Characters: — Mantle edge, antero-ventrad to 

 branchial opening smooth without specialized structures; supra- 

 anal antero-dorsad to anal opening usually widely separated; no 

 tendency toward tubular siphonal openings; inner laminae of 

 inner gill generally free from visceral mass; region just anterior 

 to pericardium of watery composition; palpi very large; mar- 

 supia — occupying two entire outer gills, when gravid pad-like, 

 enormous, tissue thickened at edge to permit transverse distention, 

 two water tubes present on either side of an enclosed central 

 undivided ovisac and facing outer and inner laminae, these 

 laminae very thin and delicate rupturing at the slightest scratch ; 

 glochidia usually large, spadiform, generally longer than high, 

 with a spine at each ventral tip; no well-defined conglutinates, 

 but held together in unstable masses by brownish mucus and a 

 tangle of larval threads. 



Shell Characters: — Shell thin for the most part; disk usually 

 without sculpturing; beaks usually coarsely sculptured with con- 

 centric or double-looped ridges; hinge variable, teeth completely 

 lacking, or, if present, rudimentary or peculiar; beak cavities not 

 deep as a rule ; sexual dimorphism rarely seen. 



Miscellaneous Remarks : — The members of this group have 

 a long period breeding season (bradytictic) due perhaps to their 

 origin at a time, as Dr. A. E. Ortmann considers, when a possible 

 shortening of the warm season induced them to retain their embryos 

 in the marsupia and discharge the glochidia in the spring; hence, 

 the constant and admirable adaptation of water-tubes for the 

 aeriation of the embryos in the marsupia whik being retained for 

 that time. This adaptation elevates this group from the primitive 

 one and places it more among the modern Unionidae. Even on the 

 basis of shell structure, in that the sculpturing, seen on the disk 

 of the shells of the Unioninae, is carried back up to the umbona 

 region where it is almost exclusively confined, there is sufficient 

 evidence for the more modern grouping. The inability of the 

 Anodontinae to spread their vavles very wide may account for 



