766 PROCEEDINGS OF THE NATIONAL MUSEUM*. VOL.XXH. 



heavy, wide, flattened ; pseudocardinals solid, direct, ragged; laterals 

 double in the left and single in the right valve, often with a small 

 secondary lateral below the large one in the right valve; cavity of the 

 beaks deep and compressed; dorsal scars under the hinge plate; male 

 and female shells alike. 



Animal having the inner gills the larger, generally free from the 

 abdominal sac the greater part or all of their length; marsupium 

 occupying all four of the gills throughout, the whole smooth and pad- 

 like. 



Section CRENODONTA Schliiter.i 

 (Type, Unio plicatns Say.) 



Shell more or less alate; beaks prominent; the surface of the valves 

 usually sculptured with oblique folds; posterior slope generally having 

 smaller radial plications which curve upward behind; epidermis brown- 

 ish or blackish; anterior muscle scars large, distinct, very shallow, 

 the anterior edge smooth, the rest apparently filled with roughened 

 shelly matter; posterior scars large, shallow, indistinct; escutcheon 

 large and dark. 



Animal with the gills generally large, rounded below; inner the 

 larger, usually free nearly or quite the entire length of the abdominal 

 sac, 2 the two pairs united to the mantle nearly but not quite to the 

 posterior end, having a small portion free; inarsupiuin occupying all 

 the four branchiae, forming very heavy, thick pads; labial palpi usually 

 large. 



(Group of Quadrula plicata.) 



Shell rounded to subrhomboid; plications usually strong, oblique, 

 though in occasional specimens the surface may be perfectly plane or 

 slightly concentrically sculptured. 



1 Schliiter applied the name Crenodonta (Verz. meiner Conch., 1836, p. 33) to a group 

 of Uuionidae, the first species of which was the Unio plicatus of Say, but he gave no 

 description of his group and did not designate a type. In 1853 Morch (Yoldi cata- 

 logue, p. 45) used this name without a description or a type, and it has been applied 

 to the plicate Uniones by von Martens (Biologia Centrali-Americana, Mollusca, 

 1900, p. 479). 



2 Dr. Lea found in Quadrula multiplicata the inner gills generally nearly or entirely 

 free, but in certain specimens they were wholly united. My own experience in exam- 

 ining the animals of this species exactly coincides with his, thus showing that the 

 character of the union of the inner gills with the abdominal sac, or their separation 

 from it, is not a generic character, as Agassiz believed, nor is it even of specific value. 

 Although I have examined thousands of animals belonging to the Flicata group 

 of the genus Quadrula, taken at various seasons and throughout almost the entire 

 range of the assemblage and of nearly all the species, I have never seen a speci- 

 men with ova or embryos in the gills save one in the alcoholic collection of Dr. Lea 

 in the Academy of Natural Sciences at Philadelphia. Other students of the Union- 

 id;e have found these species equally barren. It is probable that they breed only at 

 long intervals but in enormous numbers, a supposition strengthened by the fact that 

 Dr. Lea found about 6,000,000 young in the gills of a single Unio multiplicatm. 



