LES ENCHAINEMENTS DTJ MONDE ANIMAL 97 



than in the other ; I conclude therefrom that the 

 first is of an earlier epoch." 



To make this method clear, Gaudry sets forth 

 a series of examples from which I make a selection ; 

 the Echinoderms were, in the first place, fixed to 

 the submarine soil by a stem and, later on, became 

 detached ; in this same group, the calcareous case 

 which encloses them was first formed of pieces with- 

 out any order in the Cystidea of the Silurian, then 

 of pieces arranged in numerous lines in the Devonian 

 and 'carboniferous Sea-urchins, and finally in rows 

 reduced to the number of twenty in the Secondary 

 as in the existing sea-urchins. These stages of the 

 direction and of the reduction of the parts of the 

 " Test " [integument] of the Echinoderms enable us 

 to recognize each of these epochs. 



The Cephalopods enclosed in a shell, like the 

 Nautilus or the Orthoceras, characterize Primary 

 times ; in Secondary times there is found a 

 mixture of enveloping shells like those of the 

 Ammonites, and of internal or non-enveloping 

 shells as in the Belemnites. Lastly, the naked 

 Cephalopods, such as the Polyps and Cuttle fish, reign 

 almost exclusively in Tertiary times. The degree 

 of complication in the sutural line, which separates 

 the successive chambers of these Cephalopods, offers 

 valuable proof of the stages of evolution. In 

 Primary times the Nautilidae preponderate, with 

 simple or slightly sinuous lines of suture or partitions. 

 Later on came the Ammonites with partitions at 

 first simply denticulated, then with slashes of in- 

 creasing variety. Finally, towards the end of their 

 reign, several Ammonites, as if weary of their 

 H 



