and Emhryohgical Development. 365 



to the sudden appearance, as it were, of structural features of 

 which the relationship must be sought in very distantly re- 

 lated groups. It is to this specialty in the palaeontological 

 development that we must trace, for instance, the Cidarid 

 affinities of the Salem'ce, their papillae, the existence of a few 

 large primary interambulacral tubercles, the structure of their 

 apical system, and their large genital plates ; while it is to 

 their affinities with the Hemicidarida? that we must refer the 

 presence of the few larger primary ambulacral tubercles at the 

 base of the ambulacral area, and by their Diademopsid and 

 Echinidian affinities that we explain the indented imbricated 

 actinal system with the presence of a few genuine miliaries. 

 But all the structural features which characterize the earliest 

 types of the Desmosticha can in reality be traced, only in a 

 somewhat rudimentary form, even in the Cidaridaj. The 

 slight undulation of the closely packed, nearly vertical pori- 

 ferous zone is the forerunner of the poriferous zone first 

 separated into vertical arcs and then into independent arcs. 

 The limitation in the number of the rows of granules in the 

 ambulacral zone, and their increase in size, are the first 

 traces of the appearance of the somewhat larger primary 

 ambulacral tubercles of the Hemicidaridas and Salenice. 

 The existence of the smooth cylindrical spines of the ab- 

 actinal region of the test naturally leads to similar spines 

 covering the whole test in the other families of the Desmo- 

 sticha. The difference existing in the plates covering the 

 actinal system from those of the coronal plates leads to the 

 great distinction between the structure of the actinal system 

 and of the coronal plates in some of the Echinidas. 



Passing to the Clypeastridas and Petalosticha, we trace a 

 parallelism of the same kind, and readily in the successive 

 genera of fossil Clypeastroids, but often in widely separated 

 genera — the precise modifications which the poriferous zone 

 has undergone as it first becomes known to us in Echino- 

 cyamus and Fibularia, and as we find it in the most com- 

 plicated petaloid stage of the Clypeastroids of the present day. 

 We readily trace the changes the test undergoes from its 

 comparatively ovoid and swollen shape, to assume first that 

 of the less gibbous forms, next that of the Laganidaj, and 

 finally of the flat Scutellida? ; while we trace in the Echinan- 

 thidae the persistent structural features of some of the earliest 

 Clypeastroids, together with an excessive modification of the 

 poriferous zone. Likewise for the Echinoconidge we trace 

 mainly the slight modifications of the poriferous zone and of 

 the coronal plates ; and, finally, when we come to the Spatan- 

 gidas we find no difficulty in tracing from the most Desmo- 

 stichoicl of the Spatangoid genera the modifications of a test 



