152 JACKSON AND JAGGAR — STUDIES OF MELONITES MVLTIPORVS. 



this and a few other cases which are figured* or described, being how- 

 ever, the .only exceptions seen to the rale of terminal pentagonal plates 

 (above number 3, plate 2, figure 2) seen in either Melonites or Oligoporus, 

 which is somewhat remarkable, as in these two genera terminal plates 

 are figured or tabulated in this paper in more than 260 columns, all but 

 six of which were pentagonal (or hexagonal, when initial plates of column 

 3), and these exceptions were in all cases due to peculiar local conditions, 

 as described. 



Another feature shown by this specimen is the fact that heiDtagonal 

 plates, which lie next to terminal pentagons, are not necessarily members 

 of the immediately preceding column. Other cases of this variation from 

 the typical position of heptagonal plates, which are not infrequent, are 

 seen in the tables on pages 165-170. From lateral thrusts and pressure 

 a new column is introduced the initial plate normally takes on a when 

 pentagonal form and an adjacent plate takes on a heptagonal form, this 

 being a mechanical compensation for the missing side of the pentagon 

 in an area where plates are either hexagonal or the equivalent. 



A peculiar case of irregular arrangement of plates is that shown in the 

 ventral area of Melonites multiporus, plate 3, figure 16.t The ventral plates 

 1 and 2 show on their lower border a reentrant angle for the missing 

 initial interambulacral plate (compare with plate 3, figure 10), as dis- 

 cussed above ; the third column originates in a pentagonal plate, num- 

 ber 3, this being the only case observed in which this plate was not hex- 

 agonal (compare j)late 2, figure 2) ; the second i)late, H, in the third column 

 is hei)tagonal and very irregular in form; the initial plate of the fourth 

 column, number 4, originates in the fourth instead of the third row, its 

 characteristic position, and is heptagonal, its ventral termination making 

 a reentrant angle in plate H and being truncated dorsally by pentagon 5 ; 

 the other plates of the area are of the usual form. Only one other case, 

 plate 3, figure 12, has been seen in which plate 4 did not originate in im- 

 mediate contact with initial plate 3. It has been stated that the normal 

 form of median plates of the interambulacrum is hexagonal, and it is 

 worth noting that this case makes only a modification of the rule. The 

 sum total of plates 3, H, 4, and 5 gives 24 sides, which divided by four 

 gives an average of six sides to each plate. 



In a specimen in the Museum of Comparative Zoology (catalogue num- 

 ber 3021) two peculiar variations from the normal exist. In this speci- 

 men (plate 2, figure 7) the initial plate 6 of the sixth column is tetrago- 

 nal, a rare variation ; also, the heptagonal plate iZ, associated with the 

 terminal pentagon 8 of column 8, occurs not adjacent to the pentagon 



* Plate 2, figures 6 and 7 ; plate 3, figure 16; plate 5, figure 21. 



fThe specimen is ia Yale University Museum, diamond number 157, specimen B. 



