ROSS— ox EVOLUTION'. 



429 



Tripneustes veyitricosus, Ag. 

 Siolonoclypiis ravenilii^ 



A. Ag. 

 MellUa testudinata, Kl. 

 MelUta hexapora, A. Ag. 

 Encope mtcMlini, Ag. 

 JEncope emarginata^ Ag. 



HhyncTiolampas caribhceai^m , 



A. Ag. 

 Srissus coliimharis, Ag. 

 Meoma ventrosa, Liitk. 

 Plagionotus pectoi'ctlis , Ag. 

 Agassizia excentricia^ A. Ag. 

 Moera atropos, Mich. 



Trijoneustes depressus, A. Ag. 

 Stolonoclypus rotunduf, 



A. Ag. 



Mellita longifica^ Mich. 

 MelUta pacifica, Ver. 

 JEncope grandis^ Ag. 

 JEncope itnicropora^ Ag. 

 Hhyncholampas pacificus^ 



A. Ag. 

 £7'{ssus obesus, Yer. 

 JSIeoma grandis, Gray. 

 JPlagionotus nohilis, A. Ag. 

 Jigassizia scrohicidata, Yal. 

 JSlodra ciotho, Mich. 



The Isthmus must have been raised into dry land in Tertiary or 

 Post Tertiary times. It is difficult to doubt that the rising of this 

 natural barrier isolated two portions of a shallow water fauna whidi 

 have since slightly diverged under slightly different conditions. I 

 quote A. Ag. : "The question naturally arises, have we not in 

 the different Faunse on both sides of the Isthmus, a standard bv 

 which to measure changes which these species have undergone 

 since the raisinsr of the Isthmus of Panama and the isolation of the 

 two Faunas?" 



But it is not only in distinct " areas " that we find " representa- 

 tive " Groups, but they occur successively in the same area, since 

 in successive strata are found representative groups of Species, at 

 wider intervals, of Genera, and at still wider of Families. It is 

 interestinsc to note in this connection the o-radual differentiation of 

 a Sub-kingdom by the steady increase of its Families, Genera, (fee, 

 the expansion and differentiation occuring in its central and charac^ 

 teristic types, while those types of a more intermediate, synthetic, 

 or connective character, tend to become extinct unless saved by 

 some exceptional circumstance, as isolation, &c. 



No Sub-kingdom has left so good a record of itself in the Rocks 

 as the J\£olhisca^ and according to Woodward the number of 

 Families for the Formations is approximately as follows : Silurian 



