394 THE CANADIAN NATURALIST. [Vol. ix. 



Clypeaster proper, resembles at first more an Echinometra than a 

 Clypeastroid ; they all have simple poriferous zones and spines 

 and tubercles out of all porportion to the size of the test." 



Comparing in the same way the paleontological development 

 of the several families, he said : "We find that the Echinidse 

 proper, on the whole, agree well with the changes of growth we 

 can still follow to-day in their representatives, and that, as we 

 approach nearer the present epoch,. the fossil genera more and 

 more assume the structural features which we find developed 

 last among the Echinidge of the present day. Very much in the 

 same manner as a young Echinus develops, they lose, little 

 by little, first their Cidaridian affinities, which become more and 

 more indefinite, next their Didematidian affinities, if I may so 

 call the young stages to which they are most closely allied, and, 

 finally, with the increase in number of the coronal plates, the 

 great numerical development of the primary tubercles and 

 spines, and that of the secondaries and miliaries which we can 

 trace in the fossil Echini of the Tertiaries, we pass insensibly into 

 the generic types characteristic of the present day." 



He then adds : " The comparison of the genera of Echini which 

 have appeared since the Lias with the young stages of growth of 

 the principal families of Echini, shows a most striking coinci- 

 dence amounting almost to identity between the successive fossil 

 genera and the various stages of growth. This indentity can? 

 however, not be traced exactly in the way in which it has usually 

 been understood, while there undoubtedly exists in the genera 

 which have appeared one after the other a gradual increase in 

 certain flunilies in the number of forms, and a constant approach 

 in each succeeding formation, in the structure of the genera, to 

 those of the present day. It is only in the accordance between 

 some special points of structure of these genera and the young 

 stages of the Echini of the present day that we can trace an agree 

 ment which, as we go further back in time, becomes more and 

 more limited. We are either compelled to seek for the origin of 

 many structural features in types of which we have no record, or 

 else we must attempt to find them existing potentially in groups 

 where we had as yet not succeeded in tracing them. The paral- 

 lelism we have traced does not extend to the structure as a whole. 

 What we find is the appearance among the fossil genera of certain 

 structural features giving to the particular stages we are comparing 

 their characteristic aspect.. Thus, in the succession of the fossil 



