364 Prof. A. Agassiz on Palceontological 



has usually been understood, while there undoubtedly exists 

 in the genera which have appeared one after the other a 

 gradual increase in certain families 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 struc- 

 ture of these genera and the young stages of the Echini of 

 the present day that we can trace an agreement, which becomes 

 more and more limited as we go further back in time. 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 parallelism 

 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 suc- 

 cession of the fossil genera, when a structural feature has 

 once made its appearance, it may either remain as a persistent 

 structure, or it may become gradually modified in the succeed- 

 ing genera of the same family, or it may appear in another 

 family associated with other more marked structural features 

 which completely overshadow it. Take, for instance, among 

 the Desmosticha, the modifications of the poriferous zone of 

 the actinal and abactinal systems of the coronal plates, of the 

 ambulacral and interambulacral systems, the changes in the 

 relative proportion of the primary tubercles, and the develop- 

 ment of the secondaries. These are all structural features 

 which are modified independently one of the other; we may 

 find simultaneous development of these features in parallel 

 lines, but a very different degree of development of any special 

 feature in separate families. 



This is as plainly shown in the embryological as in the 

 palgeontological development. In the Cidaridas there is the 

 minimum of specialization in these structural features. In 

 the Diademopsidse there is a greater range in the diversity of 

 the structure of the poriferous zone and of the coronal plates, 

 as well as of the actinal system. There is a still greater range 

 among the Echinidas ; while among the Salenidas the modifica- 

 tions, as compared with those of the Echinidae and Diademop- 

 sidas, are somewhat limited again, being restricted as far as 

 relates to the poriferous zone and coronal plates, but specialized 

 as far as the actinal system is concerned, and specially impor- 

 tant with reference to the structure of the apical system. The 

 special lines in which these modifications take place produce, 

 of course, all possible combinations ; yet they give us the key 



