19 



gave way to Robinson, and Robinson to Brown, and Brown quickly 

 sunk to rest in the graveyards beneath the waters. 



And thus it has happened that Ammonites play a very important 

 part in telling the hours, nay the minutes, on the clock-face of Geo- 

 logical time. Given an Ammonite, it marks much more than a mere 

 type group, it points out a particular zone. So that if in boring a 

 well, or sinking a shaft, we meet with an Ammonite, we know 

 (within a very few feet) from that specimen what will be the kind 

 of sand, or clay, or stone, to be brought up in the next succeeding 

 borings. 



Connected with the Ammonites is another curious fossil, which 

 was for a long time the subject of much controversy : it is a thin 

 triangular-looking object (Aptyclius) generally cellular on one surface 

 and smooth on the other, having two of its edges nearly straight, 

 and the other curved. 



Some writers had put the whole group down as a species of shell, 

 others thought that they were parts of the internal organization of 

 Ammonites. At last a lucky blow solved the mystery ; in the 

 interior of an Ammonite, accidentally split asunder, these two 

 triangular bodies were observed reposing side by Fiq j0 



side, and fitting the opening of the Ammonite, thus 

 proving they were the folding-gates, most securely 

 strengthened, of the Ammonite's castle. 



The Ammonites as a genus first appeared in the 

 Triassic age, having been preceded by forms with 

 simpler sutures, the Goniatites, with the septa 

 folded in angular undulations, and the Ceratites in 



° Ammonites subra- 



a combination of circular and linear lines. Like the with oper- 



culum in situ, drawn 



Belemnites, the Ammonites 1 ended in the Creta- ^ten 

 ceous age, are never seen in the newer rocks, nor ^J^ 6 B £?af oper" 

 have they ever been met with living in modern culum 2 inchb y 3- 

 waters. 



Looking at the general shape of the Ammonite, you will perceive 

 that it may be roughly taken as a cylinder, which is continually 



1 Their size varies from a few lines to several feet in diameter ; large forms are 

 seen in the Lower Chalk and Portland Oolite. 



