IGO JACKSON AND JAGGAR — STUDIES OF MELONITES MULTIPORVS. 



repeated in the other area, and this remark applies to studies of varia- 

 tions in the interambulacrum generally. In the development of the 

 several areas of any given specimen of Melonites we find that the several 

 columns are added at the same or nearly the same horizontal plane (as 

 indicated by the numbers in the fourth column of the tables), but irregu- 

 larities, such as seen in the position of heptagons or point of origin of 

 columns, are not repeated in succeeding areas unless by chance, for on 

 the principle of chances similar variations in two or more areas would 

 occasionally occur.* 



The specimen of Melonites midti'porus number 3023 (page 168) in area 

 A has the same irregular position of the ninth column as specimen 3021, 

 having 5 columns on the left and 3 on the right at its origin ; also column 

 8 and two heptagons are out of place, otherwise the specimen is normal, 

 conforming to the ideal arrangement throughout both areas. 



The next tabulation is of a specimen of Melonites multiporus given to 

 Yale University INIuseum (page 168) as a slight return for many favors 

 received. In the two interambulacral areas shown there are 9 columns 

 of plates. All initial plates of columns are pentagons. The arrangement 

 of details is entirely normal except that two heptagons, next pentagons 

 5 and 7, are on the wrong side, and one of these heptagons is too far to 

 the left, as noted. Two ambulacra show 4 columns of plates at the ventral 

 termination. A specimen, number 3007 (page 168), in the Museum of 

 Comparative Zoology shows two ambulacral and two interambulacral 

 areas. Interambulacrum Chas two even-numbered columns, one row 

 too far to the left and one heptagon out of place ; otherwise the arrange- 

 ment of this specimen is entirely normal in its details. 



The next specimen tabulated is owned by Mr T. A. Jaggar (page 169). 

 Interambulacrum A has all the even-numbered columns left-handed or 

 with one more column on the right than on the left at the point of origin ; 

 also and perhaps as a correlated feature all the heptagons adjacent to 

 pentagons are on the wrong side in the several cases. Such uniform 

 variation from the normal has not been seen in an}^ other specimen of 

 an interambulacral area. The next interambulacrum, C, of the same 

 specimen is perfectly normal throughout, except that two heptagons 

 occupy an incorrect position. Ambulacrum B has four columns of 2:>lates 

 at the base, as usual. 



Specimen number 3017 (page 169) has 9 columns of plates in areas A 

 and / and 8 columns in the other 3 areas. The arrangement is normal as 

 far as visible, except that one heptagon is out of place and column 9 in 



* For this matter of variation it is desirable to study the figures and taVjles of Melonites with the 

 idea of radial variation especially in mind. The figures of Palceechinus (plate 7, figure 3G) and Ar- 

 chceocidaris (plate 8, figures 43-45) may also profitably be considered under this study of radial 

 variation. 



