176 R. T. JACKSON — STUDIES OF PAL^ECHINOIDEA. 



only one in plate 5, figure 21, which does not accord with the law of alter- 

 nation in the introduction of successively added columns. Plate 4 trun- 

 cates the dorsal border of initial plate 3 of column 3, inducing a hexagonal 

 form in this plate, as shown also in Melonites multiporus (plate 2, figures- 

 2 and 3) and Ollgoporus (plate 6, figure 25). The same condition of aff'airs 

 existed in 3 other areas (table, page 180). 



Column 5 is introduced in the second row after 4 by the terminal pen- 

 tagon 5. At its point of origin this column has two columns on either 

 side, the normal position. It has a heptagonal plate on its left ventral 

 border. Melonites giganteas is later discussed as an extreme member of the 

 genus on account of its great number of columns of plates, and in this early 

 introduction of the fifth column there is an interesting bit of correlative 

 evidence. Extreme types are quite commonly liighly accelerated in their 

 development, early acquiring features usually appearing a-t a later stage 

 in less specialized, more primitive allies. Cases of such acceleration are 

 seen in Nautilus, BaculUes, Spondi/lus and Discinisca.^ Here in Melonites 

 giganteus we have the fifth column originating earlier than the same 

 column in the more primitive f types, Melonites multiporus (plate 2, figure 

 2) and Oligoporus coreyi (plate 6, figure 25). 



The sixth column is introduced by the terminal pentagon 6 (plate 5^ 

 figure 21). At its point of origin it has 3 columns on the left and 2 on 

 the right, its usual position. A heptagonal plate, H, lies on the left 

 ventral border of pentagon 6. In two other interambulacral areas, E 

 and G, column 6 originates as in this area A; but in two areas, Cand /, 

 the column originates one column farther to the left with a heptagon on 

 the right (plate 4, figure 19, and table, page 180). The seventh column 

 begins with pentaojon number 7, having a heptagonal plate on its right 

 ventral border. This column at its point of origin lias 3 columns on 

 either side. Comparing this with the same plates in the other areas we 

 find (plate 4, figure 19, and table, page 180) the same arrangement in 

 areas G and /. In C the arrangement is the same except that the heptagon 

 is on the left side of the pentagon. In area Ethe column originates one 

 column too far to the right, so that there are 4 columns on its left and two 

 on its right, the heptagon lying on the left of the pentagon. This irreguLar 

 position of odd-numbered columns is of very rare occurrence, having been 

 observed in very few specimens in my researches on this family ,1 namely* 

 in Melonites multiporus (numbers 3023, 3021, 3016, 3004 and 2992 in the 

 tables, on pages 165-170) and a specimen of the same species observed 

 at Princeton (see page 153). It is somewhat remarkable in these 7 cases 



*As shown by Hyatt, Brown, Jackson and Beecher. 



t More primitive, according to the views expressed in this paper (see page 199).' 

 I An irregular position of column 5 is figured in Lepidesthes coreyi by Meek and Worthen. (See 

 remarks on that species, page 209.) 



