366 Prof. A. Agassiz on Palceontological 



in which the ambulacral and interambnlacral areas are made 

 up of plates of nearly uniform size, in which the anterior and 

 posterior extremities are barely specialized, to the most 

 typical of the Ananchytidae, in which the anterior and pos- 

 terior extremities have developed the most opposite and 

 extraordinary structural features. In a similar way we can 

 trace among the fossil genera of different families the gradual 

 development of the actinal plastron from its very earliest 

 appearance as a modification of the posterior interambulacral 

 area of the actinal side, or the growth of the posterior beak 

 into an anal snout, the successive changes of the anal groove, 

 the formation of the actinal labium, or the development of the 

 bourrelets and phyllodes from a simple circular actinostome, 

 the gradual deepening of the slight anterior groove of some 

 early Spatangoid to form the deeply sunken actinal groove. 

 Equally well we can trace the modifications of the ambulacral 

 system as it passes from the simple poriferous zones of the 

 earlier Spatangoids to genera in which the petaliferous portion 

 makes its appearance, and finally becomes the specialized 

 structure of our recent Spatangoid genera, such as ISchizaster^ 

 Moira, and the like. Finally, we can trace, to a certain 

 extent, the development of the fascioles, on one side, from 

 genera like Hemiaster, in which the peripetalous fasciole is 

 prominent, to genera like B?"issopsis 7 Brissus, and the like, of 

 the present day ; on the other, perhaps, or in both combined, 

 the formation of a lateral and anal fasciole from genera 

 like Micraster in Sjyatangus and Agassizia. Thus we must, 

 on the same theory of the independent modifications of special 

 structural features, trace the many and complicated affinities 

 which so constantly strike us in making comparative studies, 

 and which render it impossible for us to express the manifold 

 affinities we notice without taking up separately each special 

 structure. Any attempt to take up a combination of cha- 

 racters, or a system of combinations, is sure to lead us to 

 indefinite problems far beyond our power to grasp. 



In the oldest fossil Clypeastroids and Petalosticha, as well 

 as in the Desmosticha, we also find the potential expression of 

 the greater number of the modifications subsequently carried 

 out in genera of later date. The semipetaloid structure of 

 some of the earlier genera of Spatangoids, the slight modifica- 

 tions of some of the plates of the actinal side near the actino- 

 stome, are the precursors, the one of the highly complicated 

 petaloid ambulacra of the recent Spantangoids, the other of 

 the actinal plastron, leading as it does also to the important 

 differences subsequently developed in the anterior and pos- 

 terior extremities of the test, as well as to the modifications 

 which lead to the existence of a highly labiate actinostome. 



