68 



THE GEOLOGIST, 



inches and three-quarters : greatest diameter across the face 

 of the tooth, seven-eighths of an inch. The enamel, which 

 is perfect, and of a deep brown colour resembling the 

 polished shell of a cocoa nut, extends about an inch up the 

 tooth from the present crown which is irregularly broken, 

 completely surrounding it. The transverse section of this 

 portion of the tooth inclines to a semicircular, or rather a 

 horse-shoe figure, the angles being slightly curved outwards. 

 The enamel at the back of the tooth has a peculiar, reticu- 

 lated appearance like the body of a snail: on the inner side^ 

 a considerable portion is worn away in mastication, appear- 

 ing as if it had been scraped to a polish by the knife. 



The root is striated down the back and sides, but this cha- 

 racter is not apparent in front. 



The crown of the tooth exhibits very clearly the radiating 

 structure so characteristic of the herbivorous animals, as dis- 

 tinguished from the concentric arrangement common to those 

 which are carnivorous. 



Ammonites Zoologically and Geologically consideredo 



(Continued from p. 45.) 



Varieties from age.* — The modifications produced by age 

 amongst the ammonites, are so extended that they com- 

 pletely change the appearance of species. They have given 

 rise to the greatest errors, both in the description of species, 

 and in their positive application to Geology. AVe might 

 almost say that these modifications were true metamorphoses, 

 to which almost all ammonites are more or less subject. 

 Convinced for a long time, of this truth by many facts, I 

 wished to state it on the evidence of a great number of spe- 



* No one has yet noticed this fact. 



