DESCRIPTION OP MELONITES GIGANTEVS. 177 



that the uTegularity should have occurred in the seventh or ninth cohimn. 

 A still further irregularity of the seventh column is described under Melo- 

 nites septenarius (plate 9, figure 49). 



The eighth column originates in pentagon 8, with a heptagon on its 

 left and with 4 columns on the left and 3 on the right, thus being in its 

 correct theoretical position. Comparing with plate 4. figure 19, and the 

 table on page 180, we find that areas E and G are as described, except that 

 ^has the heptagon on the right instead of on the left. Areas (7 and I have 

 column 8 one place too far to the left, having 3 columns on the left and 4 

 on the right, and the heptagon of C is on the right. The ninth column 

 begins in pentagon 9 (plate 5, figure 21) with 4 columns on either side, 

 as in Melonites muUlporus (plate 5, figure 20) and Oligoporus (plate 6, figure 

 34). The plates below pentagon 9 in Melonites gigantem present a pecu- 

 liar arrangement unlike anything seen in any other specimen of the Palsee- 

 chini. Bordering on the pentagon ventrally are two heptagons, H H' ; 

 below JT there is a third heptagon, II'\ and these 3 heptagons with the 

 hexagon A, enclose a rhombic formed plate. A similar condition of 

 affairs exists in the same relation to pentagon 9, in areas C, E and /(plate 

 4, figure 19, and table, page 180). It is elsewhere stated that the plates 

 of the median columns are all hexagonal or its equivalent as a dynamic 

 consequence of the conditions of lateral pressure. This is an excellent 

 proof of the principle, for the extra side of one heptagon, H, compensates 

 for the loss of one side in pentagon 9 ; the two other heptagons, H' H'\ 

 by their two added sides compensate for the absence of two sides in the 

 enclosed four-sided plate. The enclosed rhombic plate, it is seen, termi- 

 nates ventrally in an angle ; the only plates normally doing this are 

 terminal pentagons.^ This plate, therefore, is considered as really the 

 first formed plate of column 9 which has become separated from its next 

 dorsal successor, which is pentagonal plate 9. Another case seen of a 

 plate being separated from its successor dorsally is that shown in the 

 ninth column of Oligoporus danse (plate 6, figure 31), where the terminal 

 pentagon has become separated from the next plate of its column. The 

 only other cases seen of plates of a column being separated dorso-ventrally 

 (except near the dorsal pole, where separation normally occurs, plate 3, 

 figure 13) are in the sixth column of Oligoporus missouriensis (plate 9, 

 figure 50) and the seventh column in area / of Rhoechinus gracilis (plate 7, 

 figure 36). 



Apparently this separation of the rhombic plate of Melonites giganteus 

 (plate 5, figure 21) is a case of slightly arrested development in the ninth 

 column — that is, after the first plate was formed, the rhombic one, no 



* Excepting, of coui'se, rhombic plates near the dorsal area, where the form is otherwise ac- 

 counted for. 



