Explanation op Plate 4. 



PiGUEE 18. — Melonites multiporus. Interambulacrum A is the area from which the 

 figure 12, plate 3, was drawn. The arrangement is very clear, on 

 account of the size of the plates. This area is peculiar in that the 

 fourth column originates much later than usual ; also a hexagonal 

 plate. A, exists in place of a lateral pentagonal plate and compensates 

 for the loss of a side in plate 4 ; otherwise the arrangement in this 

 interambulacrum is perfectly normal and serves as a type of the 

 method of growth of this area. In interambulacrum C, which is 

 preserved only at ventral portion, the fourth column originates in a 

 pentagonal plate, 4, much earlier than in interambulacrum A. This 

 different rate of introduction of the same numbered column in two 

 areas is very unusual (see tabulations, pages 165-170). Spine bosses 

 are visible in the lower part of the figure. The ambulacra B and I 

 have 4 columns of plates ventrally, a, a^, b^, b, and in later grow^th 

 new columns are added in each half-area, as usual (compare with 

 plate 2, figure 4). Saint Louis group, Subcarboniferous, Saint Louis, 

 Missouri. This specimen was in the Student Collection, Harvard 

 University, catalogue number 316, but is now transferred to the 

 Museum of Comparative Zoology, catalogue number 2990. Life size. 

 Page 147. 



Figure 19. — Melonites giganteus, Jackson. View from ventral area. (Compare with 

 plate 5, figures 21-24, for detail, ) Areas are lettered as in the table, 

 * page 180. The portion of ambulacrum B which is marked X is that 



area which is represented in plate 5, figure 24. In the same ambula 

 crum at Y, by careful scrutiny of the figure, 6 columns of plates may 

 be seen in the half-ambulacrum. The several interambulacral areas 

 show the method of introduction of columns by terminal pentago- 

 nal with adjacent heptagonal plates, as described. The plates are 

 thickly studded with spine bosses, which show as mammillate points. 

 The specimen, which is from Lower Subcarboniferous, Bowling 

 Green, Kentucky, is in the Museum of Comparative Zoology, cata- 

 logue number 2989. Nearly life size. Page 172. 



249 



