PLASTICITY AND PERMANENCY OF CHARACTERS. 313 



species recorded from each successive formation in the North 

 American rocks. 



fPanciplicati 



I. Radiati \ Multiplicati 



(, Dupliciplicati. . . . 



fSeptati. 



II. Lamellosi -j Aseptati 



L Submucronatus 



[Unici- jCrispus. 



III. Fimbriati J spinei | Lvis... 



I Duplicispinei .... 



Disjunctus 



Hungerfordi.. .. 



Striatus. 



Texanus 



Imbrex 



Suborbicularis... 



Orestes 



Divaricatus.. 



IV. Aperturati 



V. Ostiolati fOsteolati. 



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 



