SCAMMON : THE UMONID.E OF KANSAS, PART I. 307 



The outline and the uraboidal ratio of ellipsis are somewhat 

 variable but otherwise it is an extremely constant species. 

 The relation of the height to the length in adult shells aver- 

 ages about as 1 to H and there is very little departure from 

 this mean except in young shells, which vary somewhat. The 

 longest shells are males, but the males as a class are no longer 

 than females. 



Genus PLAGIOLA ( Rafinesque, 1819 ) Agassiz. 



"Shell triangular ovate, somewhat inflated, solid, with a 

 distinct and often sharp posterior ridge ; surface concentric- 

 ally sculptured ; umboidal area somewhat flattened ; peaks 

 high, sculptured with delicate, parallel, doubly looped ridges, 

 the anterior rounded, the posterior angular ; pseudocardinals 

 ragged ; laterals club-shaped, straight or slightly curved ; 

 cavity of the beaks moderate, often somewhat compressed; 

 nacre silvery ; female shells more or less inflated at the post- 

 basal region. 



"Animal with outer gills narrow in front, wide behind; 

 inner gills wide in front, posteriorly free or united to the 

 abdominal sac ; marsupium occupying the posterior part of 

 the outer gills, but not extending quite to the hinder end, 

 consisting of well-marked ovisacs which are rounded below ; 

 a distinct sulcus extends the whole length of the kidney- 

 shaped marsupium at the inside and outside at some distance 

 above its base ; mantle edge thickened and somewhat doubled , 

 in some cases toothed or fringed below." (Simpson.) 



Plagiola securis Lea. Plate LXIX, fig. 1. 



Unio securis Lea, Trans. Amer. Phil. Soc, ni, 1829, p. 437, pi. xi, 



fig. 17. 

 Unio lineolatus Conrad, New Fresh-water Shells, 1834, p. 70. 



Shell of moderate size, thick, thickest dorso-anteriorly, 

 roundly triangular in outline, much compressed, particu- 

 larly in the umboidal region. Anterior margin flatly 

 r'ounded ; ventral margin gently bowed ; posterior and dorsal 

 margin one gentle and continuous curve. Umboidal ratio, 

 approximately 0.19. Umbones much flattened, pointed, in- 

 curved. Anterior umboidal slope flatly rounded ; lateral 

 slope almost flat ; umboidal ridge sharp and continuous ; pos- 



