DESCRIPTION OF OLIGOPORUS MISSOURIENSIS. 185 



ness of the plates, although on the outer or distal side of the plates they 

 probably existed in that portion of the plate which was nearest the inter- 

 ambulacra, as in Oligoporus danee (plate 6, figure 30). A similar condition 

 of pores in the center of each half-ambulacrum is shown in the view from 

 the interior, or proximal side, of Oligoporus coreyi (plate 6, figure 25). In 

 Melonites multiporus the ambulacral pores in traversing the thickness of 

 the plates pass toward the center of the ambulacral area instead of each 

 half-area, as shown in plate 2, figure 5 (see page 141). 



In Oligoporus missouriensis about 5 ambulacral plates are apposed to 

 each interambulacral plate. The ambulacral plates are more regular in 

 outline than in any other species of the genus seen. One peculiarity not 

 seen in any other Paleozoic echinoid, is the fact that ambulacral plates 

 which lie opposite the horizontal sutures between interambulacral plates 

 are spread out in a fan-like fashion on the outer border, as shown in the 

 figure. 



Interambulacral areas measure about 4.1 centimeters in width at the 

 ambitus, narrowing toward the poles. Adambulacral plates are rounded 

 on the ambulacro-interambulacral suture. There are 6 columns of inter- 

 ambulacral plates at the ambitus, and no more are added in the dorsal 

 portion. The sixth column in area A is introduced by the pentagonal 

 plate 6, which is discontinuous from the next plate, 6', of its series, as in 

 the ninth column of Melonites gig anteus (plate 5, figure 21). Around this 

 plate, plate 6, there is an unusual arrangement of plates. A pentagonal 

 plate, P, lies on its left border, and an octagonal plate, 0, by its two added 

 sides, compensates for the loss of two sides in plates 6 and 6'. The sixth 

 column starts a second time in pentagonal plate 6' and adds a second 

 plate on the dorsal border of it. Then the sixth column dies out and is 

 seen no more in this area (see page 150). To take up the space where this 

 column has dropped out there is an enlargement and irregular arrange- 

 ment of plates in the fifth and fourth columns. The plates P' P" P"\ in 

 column 5, which should be hexagonal, take on a pentagonal form, and 

 one plate, jff, of column 5 is heptagonal and extended to the right so as 

 to cover the dorsal border of the last formed plate of column 6. Besides 

 these, to compensate for loss of sides in the pentagons, there is a hep- 

 tagonal plate, H\ in column 4 and another heptagonal plate, H'\ in 

 column 3. This is one of the most unusual irregularities seen in any 

 Paleozoic Echinoid, but it is all in accordance with the laws of growth 

 when the mechanical conditions are ascertained. Two other interambu- 

 lacral areas, E and G, not shown in the figure, have a similar dying out of 

 column 6. No other case has been seen in any type of a column originat- 

 ing comparatively early in the life of the individual and then dying out 

 after building a few plates, except as shown in Lepidesthes ivortheni (plate 9, 



