DESCRIPTION OF MELONITES GIGANTEUS. 173 



the exact center, 10 centimeters ; greatest horizontal diameter through the 

 middle of corona, 15.5 centimeters. These measurements are somewhat 

 affected by a slight dorso-ventral compression, which while reducing the 

 height exaggerates the horizontal diameter. 



The interambulacra at the widest part measure 4.1 centimeters ; at the 

 narrowest part, at the oral area, where three plates only exist, 8 milli- 

 meters. Following the curve of the area in its center, the interambulacra 

 measure 18.7 centimeters in length. At the oral end, as far as preserved, 

 there are three plates in the first row^ of four of the interambulacra. 

 Passing progressively from this point dorsally, new columns are intro- 

 duced until we find the greatest number attained, 11, at a point about 

 two-thirds of the distance to the apical pole (plate 5, figure 21). The am- 

 bulacra at the ambitus are narrow^er than the interambulacra, measuring 

 3.8 centimeters at the widest part, at which area there are 12 columns of 

 quite irregular plates. At the ventral termination the ambulacra are 1.4 

 centimeters in width, thus surpassing at this point the width of the in- 

 terambulacra. Ventrally there are but four columns of ambulacral plates 

 (plate 4, figure 19; plate 5, figure 22), as in Melonites multiporus. The 

 plates of the test are very thick (plate 4, figure 19), those at the median 

 zone measuring 8.5 millimeters in thickness. The sides of the plates are 

 slightly inclined to allow for mutual contact in a curved test, but are as 

 nearly perpendicular as the case admits. The plates of the adambulacral 

 columns are extended under the adjacent ambulacral plates, as in Melo- 

 nites multiporus (plate 2, figure 5) ; otherwise the plates of Melonites 

 giganteus show no tendency to imbrication, and with their thickness 

 emphasize a very considerable rigidity of the test. 



The interambulacral areas are very much elevated (plate 4, figure 19) 

 and present a comparatively sharp angle where the sides dip down to 

 meet the ambulacra. The ambulacra are sharply elevated in the median 

 portion, depressed on the lateral borders. The elevations of these areas 

 extend beyond those of the interambulacra in a peripheral line and give 

 the Echinus a melon-like form in a very accentuated degree. 



The two adambulacral columns of interambulacral plates are pentag- 

 onal, as in other species of the genus, and are crenulated on their outer 

 borders (plate 5, figures 21 and 22) by impact with adjacent ambulacral 

 plates, 3 of which commonly abut against each interambulacral plate. 

 The plates of the median interambulacral columns are hexagonal, except- 

 ing the terminal plates of columns as added, which are pentagonal, and 

 adjacent lieptagonal plates ; also excepting the newly added dorsal rhom- 

 bic plates and such others as are described in the detailed consideration 

 of this area. 



In the ambulacral areas the plates are, for the most part, ambiguous, 



