148 JACKSON AND JAGGAR STUDIES OF MELONITES MULTIPORUS. 



impinge upon initial plate 3 because of the intercalation of a peculiar 

 plate in column 3. 



Column 4 of plate 3, figure 12, and plate 4, figure 18, is introduced as 

 near the center as is mechanically possible with an even number of col- 

 umns, but it falls to the right of the actual center, having two columns 

 on the left and one on the right. Melonites, therefore, on the zone of 

 pentagon 4 has four columns of plates, and a similar remark may be 

 made of the progressively added columns where introduced. Continuing 

 dorsally, especial attention is called to the intercalation of new columns, 

 normally in regular alternation, 5 on the left of 4, 6 on the right of 5, 7 

 on the left of 6, etcetera. Continuing dorsally from pentagon 4, we find 

 the fifth column introduced with a terminal pentagon, number 5. This 

 column at its point of origin is in the middle, having two columns on 

 either side of it. This feature is characteristic of odd-numbered columns ; 

 they have an equal number of columns on either side. Occasional excep- 

 tions, however, exist, as noted on pages 165-170. Adjoining pentagon 5, 

 one of the plates, H, of column 4 has a seventh side added as a mechani- 

 cal compensation for the form of the pentagonal plate 5. This relation 

 of a terminal pentagon and an adjacent heptagonal phite, which is usually 

 a member of the immediately preceding column, is characteristic of the 

 form assumed when new columns are introduced above the fourth in the 

 whole family of the MelonUldas and in other Paleozoic echinoderms as 

 well. Passing dorsally again, we find a sixth column introduced by the 

 pentagon number 6, with an adjacent heptagon, H, on the left, which is 

 a member of column 5. Column 6 at its point of origin has three col- 

 umns on the left and two on the right and is therefore right-handed. 

 Continuing still further dorsally, we find column 7, which is introduced 

 by the terminal pentagon number 7, with an adjacent heptagon, H, on 

 its right. The heptagon is a member of column 6. Column 7 at its point 

 of origin has three columns on either side. 



It is seen in the above that the odd-numbered columns 3, 5, and 7 are, 

 ■when they originate, in the center, with an equal number of columns on 

 either side ; also it is seen that the even-numbered columns 4 and 6 are 

 as near the center as they can be, but fall to the right of the center, with 

 one more column on the left than on the right at their point of origin.* 

 It is further to be noted that in the ideal case heptagonal plates fall al- 

 ternatel}^ on opposite sides of the terminal pentagons in successively 

 added columns, namely, on the right in odd columns and on the left in 

 even columns, and are thus, when in their correct position, members of 



* In the reconstructions of the initial interambulacral plate (figures 10 and 11, plate 3) column 1 

 would also originate in a central position and column 2 would apparently originate with one col- 

 umn on the left in accordance with the above law of the method of introduction of new columns. 



