FOSSIL CEPHALOPODS. 



313 



the external or dorsal edges of the shell are marked 

 by an entire ridge or keel; (2) those in which the 

 back is crenated ; (3) those having the back sharp ; 

 (4) those with the back channelled ; (5) those with 

 the back squared ; and (6) those with the back round 

 or convex. 



The Lias and Oolite abound in Ammonites. 



Fig- 315- Ammonites amaltheus^ with a plan of foliation of chambers, and section 

 of back of shell. 



About two hundred species have been found in the 

 Lias strata alone, and they are so persistent that 

 thirteen zones are distinguished by the presence of 

 certain species of them, which have a limited range. 



The name of " snakestone " is given to them at 

 Whitby, where Ammonites of many kinds abound. 

 They are found in blue nodules, which, when broken 



