]^lem — Revision of the Palaeozoic Palae'echinoidea. 5 



PI. I., Figs. 2 and 3, PI. II., Figs. 4 and 5, and PI. III., 

 Fig. 6. 



Jackson and Jaggar in their article on Melonites lay down 

 as a law that " newly added columns normally alter- 

 nate to left and right as introduced, even numbered columns 

 typically appearing on the right of odd ones." * While this 

 rule may apply to some isolated and imperfect specimens, the 

 contrary becomes quite evident by examining a large and com- 

 plete collection of perfect fossils, as in Fig. 4, PI. II., where 

 the third column begins to the right of the second column in 

 (b) and (c) ; in Fig. 3 b where the second column appears to 

 the left of the first, and the fourth to the left of the third. 

 Other exceptions to the rule laid down by the aforesaid authors 

 appear in Figs. 2 b, c, d, Figs. 5 c. Figs. 6 c, d, e, Figs. 8 



a, b, c, d, e, Figs. 10 a, b, c. Figs. 12 a, b, and Figs. 13 a, 



b, c. If the rule given by Jackson and Jaggar were applica- 

 ble in all case, one should be able to determine, from a frag- 

 ment, whether the column introduced is an odd or an even 

 numbered one by its position to the left or right of the me- 

 dian line. But, since we find in specimens with all the plates 

 intact that columns very often fall to the opposite side from 

 the one established by Jackson and Jaggar, it is evident that 

 this rule is of no value in describing specimens. In Fig. 6 

 the second column introduced falls to the right in only two 

 areas, the third to the left of the second in only two fields; 

 the fourth to the right of the third in only three areas and 

 the fifth to the left of the fourth in only three areas. 



Another feature, which will not stand a critical test, is the 

 suppositon of the above mentioned authors, that new col- 

 umns are always introduced by a pentagonal plate with the 

 apex pointing ventrally or toward the oral area. This rule 

 is subject to many variations, columns being introduced fre- 

 quently by hexagonal or heptagonal plates and occasionally 

 by square plates. In Fig. 8 c the second column is intro- 

 duced by a square plate, in Fig. 8 d by a heptagonal plate. 

 In Fig. 6e the fifth column is introduced by a square plate. 

 In Figs. 7 b and c the first plate introducing a column is hcx- 



* Bull. Geol. Soc. Amer.jVul. 7, p. 145. 



