ENTOMOSTKACA OF MINNESOTA. 297 



margin broad, rounded at the angles, and expanded into a flattened 

 lamina or flange. Ventral margin straight for about two-thirds of its 

 length, then bending upwards at an obtuse angle. Dorsal margin 

 arched, almost angular in the middle, whence it slopes steeply to the 

 anterior border and with a bold curve towards the posterior extremity, 

 joining the ventral margin at an acute angle. Seen from above, the 

 carapace is ovate, pointed in front, and rounded behind; the greatest 

 breadth in the middle. The ventral surface is bounded by two con- 

 spicuous, elevated, arcuate ridges, one at each valve, which together 

 enclose a flattened lozenge-shaped area. Parallel to the contact 

 margin of each valve runs another straight but much less conspicuous 

 ridge, which towards the front curves outward, and joins the external 

 ridge at an acute angle, the union of the two forming a slight eleva- 

 tion, from which a single ridge runs forward, gradually merging in 

 the flattened encircling flange of the anterior border. Seen from the 

 front, the shell is sub-triangular, the sides convex, but flattened, the 

 dorsum forming a flattened arch; the ventral border perfectly flat, 

 and bounded by the projections of the outermost ventral ridges; the 

 inner ridges form also conspicuous projections, which run obliquely 

 upwards and inwards towards the flattened anterior flange. Lucid 

 spots, about six, oblong and irregularly placed, their long diameters 

 pointing transversely across the valve. Surface of the shell smooth 

 and shining, marked by fine impressed lines forming a pattern simi- 

 lar to the imbrications of fish-scales. Color pale greenish or white, 

 transparent, with large and irregularly spread patches of deep olive- 

 green or black. 



"Female. — Sub-quadrangular. Anterior border flattened, flanged, 

 forming with the ventral margin a well-marked angle; rounded above. 

 Dorsal margin boldly arched, highest at the posterior third, whence 

 it sweeps round with a deep curve to the postero-ventral angle. 

 Ventral margin straight, slightly rising behind, and terminating in a 

 flattened squamous plate, which projects backwards, with the appear- 

 ance of a spine. The other aspects are similar to those of the male 

 shell, except that the ventral surface of the left valve has at the pos- 

 terior extremity of its contact margin a flattened squamous plate, 

 which is received into a corresponding depression of the opposite 

 valve.'' 



Professor Brady (18) states that the branchial plate of the mandib- 

 ular palp arises from the lower side of the palp, thus differing con. 

 siderably from other Cyprididce. Professor Vavra(22i), however, says 

 that the branchial plate arises from the upper side of the mandibular 

 palp. 



"The first foot in both sexes terminates in three curved claws, the 

 central one being the strongest and longest. * * * * * >i< ;i< 



