TABULATIONS OP PLATE ARRANGEMENT. ' 157 



in a table gives the total number of columns of plates in the area. Un- 

 less the area is nearly or quite complete dorsally this number is omitted, 

 for if the area is incomplete any number stated might be too low. In 

 the second column the number of the column is given; in the third the 

 form of the plate in which the column terminates ventrally, indicated by 

 Pif pentagonal and by Xif hexagonal, or if otherwise by reference to 

 foot-notes. In the fourth column is given the number of the horizontal 

 row counting from the oral end in which the given column of plates 

 originates. There are two plates only at the oral end of an adult, and 

 these are called row number 1, and succeeding rows are counted from 

 that point upward (compare with plate 2, figure 2). Obviously in most 

 cases the oral termination is incomplete. In such cases the position of 

 the first introduced column is assumed to be in the average position ; 

 but the affixed number is then indicated by an asterisk, excepting when 

 only one row is wanting and the interambulacrum begins with the initial 

 plate of column 3. The advantage of this is to give the relative position 

 above the first observed column of its later added fellows. The applica- 

 tion of this notation is seen in the table of specimen number 3010. The 

 fifth and sixth columns indicate the number of columns on the left and 

 right of a column at its ventral termination, or when first introduced. 

 The seventh column gives the position of heptagonal plates whether on 

 the left or right of terminal pentagons. Any special exceptions are indi- 

 cated in foot-notes. For the sake of an easy comparison of the several 

 columns and to show individual variation, the characters which conform 

 to the ideal law of growth are printed in Roman characters ; those which 

 are variations from the ideal or normal are printed in italics, so as to be 

 readily picked out. It may be stated that the specimens were not selected 

 otherwise than as being sufficiently perfect to show satisfactorily the fea- 

 tures of plate arrangement. More tabulations might have been included, 

 but they were omitted for the sake of brevity. It will therefore, we think, 

 be conceded that the variations from the normal ideal type are relatively 

 few. A comparison of the arrangement indicated in these tables should be 

 made with the similar table of Melonites giganteus in the succeeding paper 

 and with the figures of the several genera and species figured in the plates ; 

 also with the ideal reconstruction (see figure 1, page 164). 



Remarks on the Tables of Plate Arrangement. 



The first case (page 165) is specimen 3016 * in the Museum of Compar- 

 ative Zoology. This specimen is remarkable in that it has the 10 ambu- 

 lacral and interambulacral areas preserved almost in their original com- 



* This specimen was purchased of Ward, of Rochester, for the Student Paleontologieal Collection, 

 but since its value was ascertained, it has been transferred to the Museum. 



