238 K. T. JACKSON — STUDIES OP PAL.EECHINOIDEA. 



of the corona, and in later growth these drop out to 2 columns. This 

 looks like a derivation from some Paleozoic form with man}' columns of 

 interambulacral plates, but the genus is so little known that any conclu- 

 sions with 23resent knowledge would be hazardous. 



Mr Agassiz, in his " Revision of the Echini," page 645, in writing of the 

 Paleozoic Echini, says : " No writer thus far has as yet succeeded in homol- 

 ogizing these Echini with our recent Echini ; the structure of the ambu- 

 lacral and interambulacral system finding no parallel api)arently in any 

 of the recent Echini." Farther on, at page 650, he says of the Paleozoic 

 Echini : " We know nothing as yet of the mode of formation of the addi- 

 tional rows of plates either in the ambulacral or interambulacral system ; 

 this would throw great light on the homologies of this suborder." It is 

 hoped that the present contribution may be accepted as solving some of 

 the difficulties. A study of young specimens is most important for a 

 positive statement in regard to the changes passed through in the on- 

 togeny of the individual, which has here been traced by stages in nearly 

 or quite adult specimens. Such young specimens I have been unable 

 to secure, and they must be extremely rare, if ever found. 



A PROPOSED NEW CLASSIFICATION OF PALEOZOIC ECHINI, WITH A SYSTEMATIC 



TABLE OF THE SEVERAL GROUPS. 



Basis of the classification. — From the results of preceding studies it is 

 felt that a natural systematic classification of Paleozoic Echini can be 

 based on the features of the anatomy and development of the ambula- 

 crum and interambulacrum and the relations of the peristome to the 

 ventral border of the corona. While these features are the main ones,, 

 others are considered in the minor divisions of the group, such as imbri- 

 cation of plates, the form of the plates and the position of ambulacral 

 pores. 



This classification, which is presented in tabular form, is distinctly in- 

 tended as a systematic arrangment, in which the species, genera, families 

 and orders are arranged so as to express their structural relations in a. 

 natural order. It is not intended as a phylogenetic table. While the- 

 phylogeny would probably follow somewhat similar lines, there are so 

 many great gaps that it could not be stated that this is the true genea- 

 logical history of the group. In the table details of structure are given 

 for each division from class to species, so that in this consideration of the 

 table only additional and general features will be discussed. 



SubclcLSS Palxechinoidea — Order /, Bothriocidaroida. — The simplest 

 known member of the Paleozoic Echini is the genus Bothriocidaris, as 

 discussed in previous pages. Its order should therefore form the first 

 division of its subclass. The two species known have essentially the 

 same characters. 



