106 ESSAY ON CLASSIFICATION 



Halcyonoids, the early prevalence of Astraeoids, and the very late in- 

 troduction of Madrepores, would already exhibit a correspondence 

 between the rank of the living Polyps and the representatives of that 

 class in past ages, though we may hardly expect a very close coinci- 

 dence in this respect between animals the structure of which is so 

 simple. The prevalence of Rugosa and Tabulata in the oldest depos- 

 its appears in a new light, since it has been known that the Tabulata 

 are Hydroids, and not genuine Polyps. 



The gradation among the orders of Echinoderms is perfectly plain. 

 Lowest stand the Crinoids, next the Asterioids, next the Echinoids, 

 and highest the Holothurioids, Ever since this class has been circum- 

 scribed within its natural limits, this succession has been considered 

 as expressing their natural relative standing, and modern investiga- 

 tions respecting their anatomy and embryology, however extensive, 

 have not led to any important change in their classification, as far as 

 the estimation of their rank is concerned. This is also precisely the 

 order in which the representatives of this class have successively been 

 introduced upon earth in past geological ages. Among the oldest 

 formations we find pedunculated Crinoids only, and this order re- 

 mains prominent for a long series of successive periods; next come 

 free Crinoids and Asterioids; next Echinoids, the successive appear- 

 ance of which since the Triassic period to the present day coincides 

 also with the gradation of their subdivisions, as determined by their 

 structure; and it was not until the present period that the highest 

 Echinoderms, the Holothurioids, have assumed a prominent position 

 in their class. 



Among Acephala there is not any more uncertainty respecting the 

 relative rank of their living representatives than among Echinoderms. 

 Every zoologist acknowledges the inferiority of the Bryozoa and the 

 Brachiopods when compared with the Lamellibranchiata, and among 

 these the inferiority of the Monomyaria in comparison with the 

 Dimyaria would hardly be denied. Now if any fact is well established 

 in Palaeontology it is the earlier appearance and prevalence of Bryo- 

 zoa and Brachiopods in the oldest geological formations and their 

 extraordinary development for a long succession of ages, until La- 

 mellibranchiata assume the ascendancy which they maintain to the 

 fullest extent at present. A closer comparison of the different fami- 



