SCAMMON : THE UNIONID^ OF KANSAS, PART I. 319 



PTYCHOGEN^. 



"Male and female shells essentially alike; embryos con- 

 tained in distinct ovisacs with rounded bases, occupying the 

 entire outer gills, which, when gravid, consist of a series of 

 folds." (Simpson.) 



Genus PTYCHOBRANCHUS Simpson, 1900. 



"Shell triangular, solid, sometimes becoming arcuate in 

 old specimens ; umboidal region rather elevated ; beak sculp- 

 ture consisting of faint, somewhat broken ridges, which have 

 a tendency to be doubly looped ; posterior ridge round but 

 well developed ; epidermis usually painted with wavy hair- 

 line rays or broken radiating bars, which show a tendency 

 to form square spots ; hinge plate rather wide and flat ; 

 pseudocardinals small, low, triangular, and roughened ; lat-' 

 erals club-shaped, remote; cavity of the beaks shallow; 

 muscle scars rather deep. 



"Animal with inner gills free all or part of their length 

 from the abdominal sac ; marsupium occupying the basal 

 half of the whole length of the outer gills and hanging in 

 from six to twenty beautiful folds ; ovisacs distinct, each 

 ending below in an enlarged, rounded bulb with a colored 

 spot in its center; mantle thin, with a dark, thickened bor- 

 der; branchial opening large, with very minute papillae or 

 crenulations, sometimes smooth ; anal opening crenulate or 

 smooth." (Simpson.) 



Ptychobranchus phaseolus Hildreth. Plate LXXII, fig. 2. 



Unio phaseolus Hildreth, Amer. Jour. Sci. and Arts, xiv, 1828, p. 283. 



Unio planulatus Lea, Trans. Amer. Phil Soc, ill, 1830, p. 431, pi. ix, 

 fig. 13. 



Unio camelus Lea, Trans. Amer. Phil. Soc, v, 1834, p. 102, pi. xv, 

 fig. 45. 



Shell of moderate to large size, compressed, elongate ellip- 

 soid in the outline, very heavy. Anterior margin very decid- 

 edly rounded ; ventral margin gently bowed or straight ; pos- 

 terior margin roundly pointed, the point being directed ven- 

 trally ; dorsal margin oblique and curved, generally passing 

 gradually into the posterior margin. Umboidal ratio, about 

 0.25. Umbones low and compressed, marked with a number 



