4 Trans. Acad. ScL of St. Louis. 



position indicated in iig, 11, pi. 3, to that shown in fig. 10, 

 pi 3? * It is plain that the form of the plates was constant 

 throuo;hout the existence of the individual and that growth 

 took place by the enlargement of the individual plates and by 

 the addition of plates between the apical disk and the already 

 existing plates. 



The next point, which may properly be discussed at this 

 stage, is the method of plate arrangement and the introduc- 

 tion of new columns. No adequate rule can be laid down as 

 to the normal arrangement of plates and the interval between 

 the introduction of new columns, as it is almost impossible to 

 find two specimens, or even two fields of the same specimen, 

 with the identical plate arrangement. A different rate of 

 development in two adjacent areas is not an uncommon thing, 

 being the rule rather than the exception. As an example take 

 the fields of fig. 13, pi. 6, which is a perfect specimen. In 

 section (a) the first column is introduced in the third row, 

 in section (b) in the sixth and in (c) in the fifth row. The 

 second column begins in (a) in the sixth, in (b) in the ninth 

 and in (c) in the eighth row. The third column starts in (a) 

 in the tenth, in (b) in the ninth and in (c) in the twelfth 

 row. The fourth column is introduced in the thirteenth row 

 in (a), in the twelfth in (b) and (c). In Fig. 8, PL IV the 

 first column begins in the second row in (a), (b), (c), (e) 

 and in the fifth in (d). The second column starts 

 in the sixth row in (a), (b), (e), and (c) and in 

 the fifth in (d). The third column is introduced in the 

 ninth row in (a), (b), (e) and in the sixth in (c) and 

 (d). The fourth column begins in (a) and (b) in the twelfth 

 row, in (c) in the sixth, in (d) in the ninth and in (e) in the 

 thirteenth row. The fifth column is introduced in the 

 fifteenth row in (a), (b) and (e), in the ninth in (c) and in 

 the twelfth row in (d). The sixth column has its beginning 

 in the eighteenth row in (a), in the nineteenth in (b) and 

 (e), in the eleventh in (c) and in the fourteenth in (d). 

 Other specimens showing this feature plainly are figured on 



* Jackson and Jaggar. Bull. Geol. Soc. Amer., vol. 7. 



