Natural History of the United States. 207 



of the fossils incorrectly appreciated, I shall therefore limit my- 

 self here to a general comparison, which may, however, be suffi- 

 cient to show, that the improvements which have been introduced 

 n our systems, upon purely zoological grounds, have nevertheless 

 tended to render more apparent the coincidence between the re- 

 lative standing among living animals and the order of succession 

 of their representatives in past ages. I have lately attempted to 

 show, that the order of Halcyonoids, among Polyps, is superior 

 to that of Actinoids ; that, in this class, compound communities 

 constitute a higher degree of development, when contrasted with 

 the characters and mode of existence of single Polyps, as exhibit- 

 ed by the Actinia ; that top-budding is superior to lateral bud- 

 ding ; and that the type of Madrepores, with their top-animal, or 

 at least with a definite and limited number of tentacles, is 

 superior to all other Actinoids. If this be so, the prevalence of 

 Actinoids in older geological formations, to the exclusion of 

 Halcyonoids, the prevalence of Rugosa and Tabulata in the 

 oldests deposits, the later prevalence of Astraeoids, and the very 

 late introduction of Madrepores, would already exhibit a corres- 

 pondence between the rank of the living Polyps and the repre- 

 sentatives of that class in past ages, though we may hardly ex- 

 pect a very close coincidence in this respect between animals the 

 structure of which is* so simple. 



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 circumscribed within its natural limits, this succession 

 has been considered as expressing their natural relative standing, 

 and modern investigations respecting their anatomy and embry- 

 ology, however extensive, have not led to any important change 

 in their classification, as far as the estimation of their rank is con- 

 cerned. This is also precisely the order in which the representa- 

 tives of this class have successively been introduced upon earth in 

 past geological ages. Among the oldest formations we find pe- 

 dunculated Crinoids only, and this order remains prominent for 

 a long series of successive periods ; next come free Crinoids and 

 Asterioids ; next Echinoids, the successive appearance of which 

 since the triasic period to the present day, coincides also with the 

 gradation of their subdivisons, as determined by their structure ; 

 and it was not until the present period, that the highest Echino- 

 derms, the Holothurioids, have assumed a prominent position in 

 their class. 



