REMARKS ON TABLES OF PLATE ARRANGEMENT. 159 



the corona in two very perfectly preserved and quite average specimens 

 in all details. The odd-numbered columns are all median at their point 

 of origin. Six even-numbered columns are out of place, originating 

 one row too far to the left ; also 6 heptagonal plates occupy the incorrect 

 position. Otherwise the specimen conforms entirely to the ideal method 

 of arrangement as here formulated. 



The specimen (number 2991) next considered (page 167) is in the 

 Museum of Comparative Zoology. It has 4 columns of ambulacral plates 

 ventrally in the 4 areas shown ; also there are 3 interambulacral areas 

 which are preserved from the second row of plates nearly to the dorsal 

 termination of the areas. The initial plates of 14 columns shown are 

 pentagonal. The initial plate of column 3 in area Q is hexagonal, but 

 in areas A and E the ventral border of the initial plate 3 is not shown ; 

 therefore, though doubtless hexagonal, the form is not given. The several 

 columns in each area originate in about the usual row as compared with 

 each other and with other specimens. The 9 odd-numbered columns are 

 all median in position at the point of origin. Four even-numbered 

 columns are out of place, being one column too far to the left ; also 5 

 heptagons are in the incorrect position, by rule ; otherwise the plate ar- 

 rangement is entirely normal throughout. 



Specimen number 3021 (page 167) in the Museum of Comparative 

 Zoology has 9 columns in both areas shown. Area A is complete dorsally ; 

 one column, the eighth, is one column too far to the left at its point of 

 origin, but otherwise this area conforms to the ideal in all details of its 

 plate arrangement. Area 0, however, is one of the most irregular areas 

 met with in any specimen. It is figured in plate 2, figure 7. The 

 ninth column of this area has 5 columns on the left and 3 on the right 

 at its point of origin. This is a very unusual variation, for odd columns 

 are almost universally median in position. A few similar cases will be 

 noticed in other tables. The sixth column in the same area, C, termi- 

 nates ventrally in a tetragonal plate, which is a very rare variation ; only 

 four other similar cases have been met with in any Palseechinoid. One 

 heptagon, that next pentagon 9, is out of place. The heptagon which 

 should come next to pentagon 8 lies in the correct horizontal row, but 

 exists as a member of column 5 instead of column 7. This curious va- 

 riation of position of the heptagon has been observed in only one other 

 case in Melonites multiporus. It has been observed, however, in Melonites 

 giganteus (see the table of that species, area /, column 11). Interambu- 

 lacrum ^, it is interesting to note, is almost entirely normal throughout, 

 only one even-numbered column being out of place. The columns of 

 plates are introduced at the same row in each area. Comparing the two 

 areas of this specimen, it is seen that the irregularity of one area is not 



XXII— Bull. Geol. Soc. Am., Vol. 7, 1895. 



