REVISION OF THE KEYHOLE URCHINS^ — CLARK 437 



half of test much thicker than posterior, the slope from 

 margin to apex being usually quite abrupt; periproct com- 

 monly more or less circular (except in loiujiflssa) ; un- 

 paired lunula very long, usually 25^0 percent of test length. 

 &*. Unpaired lunule of interambulaci-um 5 long and narrow, 25 to 

 40 percent of test length, its breadth less than 10 percent of 

 its length ; ambulacral intermediate areas I and V on oral 

 surface long and narrow, greatest width less than 40 percent 



of length; peripx-oct much longer than wide longifissa 



6'. Unpaired lunule usually less than 30 percent of test length, its 



breadth 15 to 20 percent of its length or more; ambulacral 



intermediate areas I and V on oral surface with their 



greatest width more than 40 percent of length ; periproct 



more nearly circular. 



c\ Ambulacral intermediate areas on oral surface notably wide, 



in II and IV the width is more than half the length; 



heads of the minute, capitate, dorsal primaries of nominal 



size ; length of test 85 to 91 percent of width latiambulacra 



c^ Ambulacral intermediate areas on oral surface not unusually 

 wide, in II and IV the width is less than half the length ; 

 heads of the minute dorsal primaries much enlarged; 

 length of test 80 to 85 percent of the width lata 



The new forms diagnosed in the above key may be described as 

 follows : 



Genus MELLITA Agassiz 



MELLITA LATA.i new species 



Plate 60, Figure 1 ; Plate 61, Figure 1 ; Plate 62, Figltees 1, 2 



Test rather stout, especially anteriorly, 70 mm. long by 87 mm. 

 wide and 8 mm. high at apex, 20 mm. back of the anterior margin ; 

 posterior margin only 2 mm. thick; the slope up from anterior 

 margin to apex is somewhat abrupt. Center of abactinal system 

 back of apex, 32 mm. from anterior margin; genital pores four. 

 Petaloid areas 44 mm. long and about 42 mm. wide; unpaired petal 

 (III) 18 mm, long (from ocular pore to tip) by 10 mm. wide, nar- 

 rowed at tip but not closed; petals II and IV, 17 by 12 mm., widest 

 distal to middle, narrowly open, the interporiferous areas curved 

 backward a trifle as if to join the anterior lunules; petals I and V, 

 22 by 12 mm., widest near the narrowly open tip. All petals very 

 blunt. Lunules II and IV about 20 mm. long by 2 mm. wide, 

 distinctly curved with the concavity anterior; lunules I and V a little 

 shorter, wider, and less curved; unpaired lunule 24 mm. long and 

 more than 3 mm. wide, reaching to within 7 mm. of the posterior 

 margin of the test. Periproct about 3 mm. long by 2.5 mm. wide, 

 its anterior margin 3 mm. posterior to the rear margin of the 



iLn/ws = broad, in reference to the extraordinary width of this species. 



