160 The Life-Historii of Spirifer herit<. 



The study of these Spirifers, in tlieir historic relations, fur- 

 nishes evidence of the persistence of specific characters in a vari- 

 able condition, for Avhich the limits of variation seem to be 

 already fixed in the primitive form. The prominent primitive 

 varieties appear distinctly here and there along the geological 

 periods marking the life of the species, but neither ])ass out of 

 existence nor become materially modified. 



The length of time is from the Upper Silurian, near the be- 

 ginning of the life of the genus, to the Carboniferous, and it may 

 be beyond, — extending over nearly three-quarters of the time in 

 which the genus lived. 



The following is a tabular view of the relations of the Silurian 

 and Devonian forms of which Spirifer ci'ispus of the Niagara 

 in New York is the type ; the tracing of the history through 

 the European forms and higher into the Carboniferous is re- 

 served for further study. In the table, lateral extension is ex- 

 pressive of the morphological variations ; each line rei)resents 

 one of the geological formations, which are arranged in their 

 natural order ; and the name of each species is placed in the 

 position on the line representing its supposed relation to the 

 typical form of ^S'. vrisptis. 



Chemung ' prematurvis 



Portage -.. laevis 



Hamilton fimbriatus. subumbona 



Corniferous j fimbriatus 



Oriskany tribulis 



r N. Y. & Tenn. . . Saffordi (pars. ) 



T TT 1 1 1 Maryland octocostatus modestus 



Lower Helderberg j ^^^^ York .....cyclopterus (pars.) 



[New York, Vanuxemi 



Niaa-ara ' '"^^^^^^ -" "- " crispus 



° \ limestone sulcatus (pars).. .crispus bicostatus 



