PLASTICITY AND PERMANENCY OF CHARACTERS. 3 1 J. 



species recorded from each successive formation in the North 

 American rocks. 



fPanciplicati. 

 I. Radiati \ Multipl'cati. 



Dupliciplicati.. 



f Septati 



II. Lamellosi -j Aseptati 



1^ Submucronatus 



fUnici- jC"^P"^- 

 III. Fimbriati \ ^P'"^M Laevis... 



[ Duplicispinei . ... 



IV. Aperturati \ 



Disjunctus 



Hungerfordi.. .. 



j Striatus 



Texanus 



Imbre.x 



Suborbicularis. .. 



Orestes 



Divaricatus 



V. Ostiolati f Osteolati 



J 



I Acuminatus.. .. 

 VI. Glabrati Aseptati 



Each Type of Spirifer shows a Continuous Series of Species. 

 — After making allowance for the gaps to be expected in our 

 limited knowledge of fossil faunas, it will be seen, by a glance 

 at the table, that each of these special types of Spirifer had 

 a more or less continuous series of species, persisting for two 

 or three or a dozen formations, represented, not by the same 

 form, but by mutations of the earlier form which differ suffi- 

 ciently from it to call for different specific names in the suc- 

 cessive formations. Some of the species are reported for 

 two contiguous or for several consecutive formations, but 



