EEMARKS ON TABLES OF PLATE ARRANGEMENT. 163 



7 cases in the eleventh row ; the sixth column, therefore, in the majority 

 of cases originates in the tenth row. The seventh column in 4 cases origi- 

 nates in the twelfth row, in 15 cases in the thirteenth row, in 11 cases in 

 the fourteenth row, in 8 cases in the fifteenth row, and in one case in the 

 seventeenth row ; the seventh column, therefore, originates in the great 

 majority of cases in either the thirteenth or fourteenth row, but occa- 

 sionally lower or higher. The eighth column originates in the sixteenth 

 row in 5 cases, in the seventeenth row in 7 cases, in the eighteenth row 

 in 5 cases, in the nineteenth row in 5 cases, in the twentieth row in 3 

 cases, and in the twenty-second row in one case ; there is considerable 

 variation, therefore, in the period of introduction of the eighth column 

 without special preponderance in favor of any one row ; the sixteenth to 

 the nineteenth row may be taken as the average swing in rate of appear- 

 ance of this column. The ninth column has a similar wide swing in its 

 period of introduction ; it originates in the twentieth row in 3 cases, in 

 the twenty-first row in three cases, in the twenty-second row in 2 caseS) 

 in the twenty-third row in 3 cases, and in the twenty-fifth row in one 

 case ; this ninth and last column added, therefore, makes its appearance 

 in a majority of cases in the twentieth to the twenty-third row. 



The interesting result of this tabulation of periods of appearance of 

 columns is to show with what regularity the several columns were intro- 

 duced in the development of the animal, all appearing within the close 

 limits of a perfectly definite period in growth. As might be expected, 

 the columns added first, from 1 to 4, inclusive, are the most constant in 

 their rate of appearance, later added columns being more variable, yet a 

 pretty close adherence to the abstract law of specific periodicity is main- 

 tained in all the columns as added. Comparing this rate of introduction 

 of columns with that of Melonites giganteus, we find most suggestive and 

 interesting conclusions, as discussed under the consideration of that spe- 

 cies in the succeeding paper. 



Turning to the results obtained in other columns of the tables, we would 

 again include Melonites multipor us and Melonites giganteus in one considera- 

 tion of the details. Out of a total of 138 odd-numbered columns intro- 

 duced, all but 6, or 95+ per cent, originate in the middle, with an equal 

 number of columns on either side. In these 6 exceptions the column 

 had one more column on the left than on the right, or was right-handed- 



In the cases of 113 even-numbered columns 82 originated with one more 

 column on the left than on the right, and 31 originated with one more 

 column on the right than on the left ; that is, 72+ per cent were in the 

 theoretically correct position, or left-handed. In no case of column ar- 

 rangement in either odd- or even-numbered columns was the newly intro- 



