244 The University Science Bulletin. 



edges of each tooth are more or less parallel and the apical edges taper, giving 

 a gabled appearance to the tooth; the caudal edge is somewhat longer than 

 the cephalic, fairly regular as to size, shape and spacing, though not entirely so; 

 primary teeth bear small secondary teeth on caudal edge more commonly, 

 and a few teeth also bear secondary teeth on the cephalic edge; tip notched 

 with small teeth on both dorsal and ventral edges, continuous around the tip, 

 nineteen present on ventral edge between preapical prominence and apex; 

 area of ducts inconspicuous, the apices and circular openings alone being 

 visible; open along ventral ai)ical edge, at extreme apex, along dorsal edge of 

 toothed area, and irregularly in the basal portion of the valve; the two valves 

 of the pair are joined one to the other by an indistinct, chitinous connection 

 present on the dorsal edge at about a third of the length. 



Platymetopius cinereus Osborn. 



(PI. XXX, fig. 4.) 



Length, 1.13 mm.; greatest width, 0.13 mm. Curved basal connection nar- 

 row and rodlike; a wider area extends almost to midpoint, a slighter wider 

 apical portion occupies the rest of the valve; no preapical prominence; 

 slightly curved, tip narrowed, ends in obtuse-angled point, chitinization mod- 

 erately light; strengthening rod extends caudad as far as sixth dorsal tooth 

 from apex. Toothed area on dorsal edge occupies a little more than the apical 

 half; teeth twenty-three to twenty-four in number, moderately large, of a 

 general triangular shape with the caudal side in most cases longer than the 

 cephalic and with apices rounded, fairly regular in size, shape and spacing; 

 bears small secondary teeth on caudal edge, three to eight in number, and a few 

 of the teeth also bear a single secondary tooth on the cephalic edge; tip 

 notched with small teeth on both dorsal and ventral edges, more numerous and 

 distinct on ventral edge, not continuous around the tip, fourteen present on 

 the ventral edge; ducts very inconspicuous, only a few being visible in the 

 apex of the valve, apices of ducts and duct openings visible ; open along ventral 

 apical edge, at extreme apex, along dorsal edge of toothed area, and irregu- 

 larly in the basal portion of the valve; the two valves of the pair are joined 

 one to the other by a poorly defined, heavily chitinized connection present on 

 the dorsal edge at about one-third the length. 



Platyjnetopius frontalis Van Duzee. 



(PI. XXIII, fig. 6; pi. XXX, fig. 5.) 



Length, 1.26 mm.; greatest width, 0.16 mm. Curved basal connection nar- 

 row and rodlike; a slightly wider portion extends to a point nearly midway; 

 the apical portion is slightly wider, naiTOws caiidad to apex, bears a suggestion 

 of a broadly rounded preapical prominence on the ventral edge; shghtly 

 curved, tip narrowed, ends in a blunt, obtuse-angled point, chitinization mod- 

 erately heavy, heavier than in P. acutus and P. cinereus; strengthening rod 

 extends caudad as far as fifth dorsal tooth from apex. Toothed area on dorsal 

 edge occupies a little more than the apical half; teeth twenty-one to twenty- 

 two in number, rather large; in the greater number the basal edges of each 

 tooth are parallel and the apical edges taper to a rounded apex, fairly regular 

 in size, shape and spacing ; bear small secondary teeth for the most part on the 



