No. 7.] AMERICAN ASSOCIATION. 395 



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 succeeding genera of the same family, 

 or it may appear in another family, associated with other more 

 marked structural features which completely overshadow it." 



Summing up, he says: -'We may, 'however, in a very genera^ 

 way, state that we know the earliest embroyonic stages of the order 

 of Echinoderms of to-day, which, with the exception of the 

 Blastoidea and Cystideans, are identical with the fossil orders, 

 and that as far as we know they all begin at a stage where it 

 would be impossible to distinguish a Sea-urchin from a Star-fish, or 

 an Ophiuran, or a Crinoid, or an Holothurian, — a stage in which 

 the test, calyx, abactinal and ambulacral systems are reduced to 

 a minimum. From this identical origin there is developed at the 

 present day, in a comparatively short period of time, either a 

 Starfish, a Sea-urchin, or a Crinoid ; and if we have been able 

 successfully to compare, in the development of typical structures, 

 the embryonic stages of the young Echini with their development 

 in the fossil genera, we may fairly assume that the same process 

 is applicable when instituting the comparison within the difi"erent 

 limits of the orders, but with the same restrictions. That is, if we 

 wish to form some idea of the probable course of transformations 

 which the earliest Echinoderms have undergone to lead us to those 

 of the present day, we are justified in seeking for our earliest 

 representatives of the orders such Echinoderms as resemble the 

 early stages of our embryos, and in following, for them- as for the 

 Echini, the modifications of typical structures. The.se we shall 

 have every reason to expect to find repeated in the fossils of later 

 periods, and, going back a step further, we may perhaps get an 

 indefinite glimpse of that first Echinodermal stage which should 

 combine the structural features common to all the earliest stages 

 of our Echinoderm embryos. And yet, among the fossil Echino- 

 derms of the oldest periods, we have not as yet discovered this 

 earliest type from which we could derive the Star-fishes, 

 Ophiurans, Sea-urchins, or Holothurians." 



" This may not seem a very satisfactory result to have attained. 

 It certainly has been shown to be an impossibility to trace in the 

 paleontological succession of the Echini anything like a sequence 

 of genera. No direct filiation can be shown to exist, and yet 

 the very existence of persistent types, not only among Echino- 

 derms, but in every group of marine animals, genera which have 



