312 KANSAS UNIVERSITY SCIENCE BULLETIN. 



As a rule there is little variability in either the outline or 

 inflation of this species, but the coloration of the epidermis is 

 subject to great variations. The ground shade may be either 

 light straw, brown, olive-gray, or almost black. The rays 

 when present may be either solid or made up of numerous 

 small V-shaped dark spots, which have given to this form the 

 name of Umo zig-zag. All gradations may be found between 

 these different types. 



Genus TRITOGONIA Agassiz, 1852. 



"Shell solid, elongate rhomboid, having a strong irregular 

 posterior ridge, obliquely truncated behind in the male ; in 

 the female this region is somewhat compressed and rounded ; 

 base incurved ; whole surface except rounded wing of the 

 females covered with pustules ; beaks low, incurved, and 

 turned forward over the well-developed lunule, which is 

 elongate and filled with epidermal matter ; beak sculpture 

 strong, consisting of irregular subparallel ridges which are 

 curved up behind, and fine radiating ridges in front and 

 behind this ; epidermis dark olive ; hinge plate rather narrow, 

 pseudocardinals strong, ragged ; laterals long and straight, 

 near to the pseudocardinals ; adductor scars shallow ; cavity 

 of the beaks rather deep and compressed ; female shell more 

 compressed than that of the male. 



"Animal with inner gills much larger than the outer, 

 generally free for the most part from the abdominal sac ; 

 palpi enormous, elongate, united to each other behind, and 

 to the mantle a part of their length ; mantle thin, with a 

 thickened, dark double border, the inner edge often toothed 

 throughout, the base much thickened at the posterior end 

 and folded at the branchial opening ; branchial opening large, 

 with numerous crowded papilhie ; anal opening smooth or 

 with only fine dentations ; superanal opening long, closed 

 below ; in the female there is a thickened flap of the mantle 

 which fills the circular posterior expansion of the shell, and 

 which has a smaller fiap inside ; foot and abdominal sac 

 large, the latter winged in front." (Simpson.) 



