Number of Animals in Geological Times. 363 



a natural genus more closely allied to Crassatella than to Unioy 

 nobody thinks any longer of looking for Unios in marine deposits. 

 As long as certain fossil fishes of the Zechstein and Lias were 

 referred to the genera Esosc and Cyprinus, the families of which 

 these genera are the types could be supposed to have extended 

 their range far beyond the tertiary formations, before which how- 

 ever no one of their representatives is to be found. Before the 

 Spatangoids were divided into natural genera, the genus Spa- 

 tangus was mentioned among the fossils of the oolitic as well as 

 the cretaceous and tertiary formations ; now it is restricted to the 

 last among the fossils and found also among the living. I do 

 not believe that a single genuine species of Gorgonia is found 

 among the fossil Polypi, and yet that genus appears in the lists 

 of fossils from the palaeozoic period to the present time. 



Since it is not my intention to enter here upon a special cri- 

 ticism of the innumerable errors of this kind still to be found 

 even in modern lists of fossils, I shall not multiply my examples. 

 These may be sufficient to show how important a correct generic 

 identification of the fossils may be, in the estimation of the order 

 of succession of organized beings ; and I cannot but lament the 

 utter want of consideration evinced even by many distinguished 

 palaeontologists in this respect, who seem to think that the know- 

 ledge of species is sufficient in itself to a proper appreciation of 

 the order of creation, and that genera are arbitrary divisions 

 established by naturalists merely for the sake of facilitating the 

 study of species, as if the more general relations of living beings 

 to one another were not as definitely regulated in all their 

 degrees by the same thinking mind, as the ultimate relations of 

 individuals to one another. 



In the third place, the natural affinities of genera should be ascer- 

 tained. Unless the genera are referred to the families to which 

 they truly belong, unless the rank of these families in their 

 respective classes is positively determined, unless the peculiarities 

 of structure which characterize them are taken as the foundation 

 of such an arrangement, and further corroborated by the mode 

 of development of their respective types, it would be a hopeless 

 task to attempt to determine the order of succession of the fossils 

 in different geological formations. Before the Crinoids which 

 Lamarck placed among the Polyps had been referred to the class 

 of Echinoderms, nobody could have understood the beautiful gra- 

 dation so fully ascertained now, which may be traced through all 

 geological formations among these animals. Before it was ascer- 

 tained that the little animal described by Thompson as a living 

 Crinoid, under the name of Pentacrinus europceus, for which 

 De Blainville established the genus Phytocrinus, is in reality the 

 young of a Comatula, nobody could have suspected the wonder- 



