24 NAIADES OF MISSOURI 



Before maturity the glochidia have been observed to be yellow- 

 ish brown and contained in pinkish sacs. 



Fusconaia undata trigona (Lea). 

 ("Little Pigtoe.") 

 PL XV. Figs. 31 A and B. 

 1913a — Fuscofiaia undata trigona (Lea) Ortmann, Pr. Am. Phil. Soc, 

 LII, No. 210. 



ANIMAIv CHARACTERS. 



The nutritive and reproductive structures of this subspecies 

 are, of course, identical with those of its species. 



SHELL CHARACTERS. 



Shell more quadrate than that of F. fiava, post-umbonal 

 ridge not so prominent, more solid anteriorly, higher fuller beaks, 

 epidermis darker. Compared to its typical species it never matures 

 to be as large, nor as heavy, is not quite so upright, nor as inflated, 

 has lower beaks and more of a reddish epidermis. The internal 

 shell structures are identical with those of its species. From the 

 subspecies, trigonoides, it differs chiefly in being more upright, 

 not so elongated, nor as large. 



Length Height Diameter Locality 



65 X 53.5 X 41mm (Osage, R., Sagrada, Mo.) 



( " " Monegaw Springs, Mo.) 



( " " Warsaw, Mo.) 



( " " Linn Creek, Mo.) 



Miscellaneous Remarks: — This sub-species is one of the 

 decided intergrades for F. undata and fiava and is the most common 

 form in the interior streams of the state — especially in the Osage 

 where the writer, in a 300 mile survey by boat, made a thorough 

 study of them to find the interior of shell and soft parts to be 

 identical with those of the species; however, none of these forms 

 were found with reddish conglutinates, nor yellowish soft parts. 

 Probably this whitish colorization was due to the advanced stage 

 of gravidity in which they happened to be found when they were 

 encountered between the upper and lower stretches of the river, 

 at the latter part of July. This variety of undata may be ecolo- 

 gically determined as a dweller in medium sized rivers, or, if found 

 in a large river, in medium stream conditions. This is found to 

 be the case in the Osage, for the more the mouth is approached 

 the more this form is supplanted by the heavier, more inflated 



