EXTENT AND NUMBER OF SPECIES. 



253 



which they are found, and vary in size from a line to more 

 than four feet in diameter.* 



It is needlees here to speculate either on the physical, or 

 linal causes which produced these curious changes of 

 species, in this highest order of the Molluscous inhabitants 

 of the seas, during some of the early and the middle ages of 

 geological chronology ; but the exquisite symmetry, beauty, 

 and minute delicacy of structure, that pervade each varia- 

 tion of contrivance throughout several hundred species, 

 leave no room to doubt the exercise of Design and Intelli- 

 gence in their construction ; although we cannot always 



SUB-GENERA OF AMMONITES. 



LIVING SPECIES. 



In Tertiary strata 



In Cretaceous system. . , . 



In Oolitic system 



In Saliferous system. . . . 

 In Carljoniferous system. . 

 t In Primary strata 



12 



4 

 22:27 



12 



26 



Total 223 species. 



" It is easy to see how important, in questions concerning the relative 

 antiquity of stratified rocks, is a knowledge of Ammonites, since whole 



sections of them are characteristic of certain systems of rocks." Phillips's 



Guide to Geology, 8vo. 1 834, sec. 82. 



* Mr. Sowerby (Min. conch, vol. iv, p. 79 and p. 81,) and Mr. Mantell 

 speak of Ammonites in Chalk, having a diameter of three feet. Sir T. 

 Harvey, and Mr. Keith Milnes, have recently measured Ammonites in the 

 Chalk near Margate, which exceeded /our /fief in diameter; and this in cases 

 where the diameter can have been in a very small degree enlarged by pres- 

 sure. 



t The strata here termed primary are those which, in the Sections, (PI. 1,) I have in- 

 cluded iu the lower region of the transition series. 



VOL. I.— 22 



