VARIATIONS IN THE INTERAMBULACRUM. 153 



but in the fifth instead of the seventh column, as usual. This heptagon, 

 it is to be noted, is in the correct horizontal row, although in an unusual 

 vertical column. Two or three similar cases of unusual position of hep- 

 tagonal plates have been observed in Melonites multiporus and Melonites 

 giganteus, Jackson (see area Jin tabulation of the latter species). 



In the E. M. Museum at Princeton, New Jersey, there is a specimen 

 of Melonites multiporus (catalogue number 1462) which has nine columns 

 of plates. The initial plate of column 5 is tetragonal with two adjacent 

 heptagonal plates, as in initial plate 6, of plate 2, figure 7. Another 

 peculiarity is the fact that column 7 at its point of origin has four 

 columns on the left and two on the right instead of being median in 

 position. An adjacent interambulacrum of the specimen is normal, not 

 showing the mentioned local peculiarities. 



A peculiar terminal plate of column 4 is shown in plate 3, figure 10. 

 In this specimen the initial plate of its column is tetragonal in form, and 

 this is almost the only case of the kind seen, except that shown in plate 

 2, figure 7, and plate 2, figure 6. This plate impinges on the dorsal border 

 of initial plate 3 in the usual manner (compare with plate 2, figure 2). 

 It is shown that newly introduced plates near the dorsal area are rhombic 

 in form, and it would seem that from the quick introduction of the second 

 plate 4' in this column the initial tetragonal plate 4 had been checked in 

 its development so as never to assume the typical form. 



While in the body of the interambulacrum only terminal ventral plates 

 of newly added columns are pentagonal, a few exceptions have been ob- 

 served in which other plates have a pentagonal form. Such a case is 

 seen in plate 3, figure 17, where, besides the initial pentagon of column 

 8 there are two accessory pentagons developed, one, P, on the left of pen- 

 tagon 8 and one, P', on the dorsal border of the same. We see nothing 

 to account for this peculiarity, which is exceedingly rare, except chance 

 variation. In this case it would seem that either pentagon 8 or P might 

 be selected as the initial pentagon of column 8, but the left-hand penta- 

 gon is evidently a member of column 7 ; also selecting the right-hand 

 pentagon for the terminal brings column 8 in its correct theoretical posi- 

 tion, with four columns on the left and three on the right. 



Melonites multiporus is described as -having seven or eight columns of 

 plates in the interambulacrum. This is the rule; but in the large series 

 of specimens examined many exceptions have been found in which 

 nine columns existed in the interambulacrum. Thirteen of such areas 

 are tabulated in the tables on pages 165 to 170. One of the best 

 examples seen is that shown in plate 5, figure 20. In the right-hand 

 interambulacrum of this specimen, columns 5, 6, 7 and 8 originate in 

 pentagonal plates, as numberedj with heptagonal plates on their left 



