18 FOSSIL MOLLUSCA OF THE CHALK. 



Ammonites are distinguished from other genera of discoidal chambered Cephalopods 

 by the branching lobes of their mantle, causing the edges of the septa to form a very 

 complicated pattern against the side of the whorl, which is seen when the surface is removed 

 or worn away. The forms of these lobes have been used, as distinguishing specific 

 characters, by Von Euch and many subsequent writers. Although I am not disposed to 

 attach so much importance to them as is sometimes done, I have, as far as possible, given 

 an outline of the lobes of the septa of each species. 



To facilitate description, Von Buch proposed to restrict the term lohe to the more sub- 

 divided and branching portions of the edges of the chamber which extend downward from 

 the mouth of the shell, and to call saddles the more rounded and less intersected divisions 

 which project upward towards the mouth ; thus the outline of a septum is made up of lohes 

 alternating with saddles. The middle of the back of the whorl is occupied by the dorsal 

 lohe, opposite to which is the ventral lohe, which, abutting against the previous whorl, is 

 always concealed : between these are the lateral lobes and saddles. References to these 

 names will be found in the explanation of Plate VH. 



In using these characters, the observer must bear in mind that they vary at different 

 ages of the animal, and are therefore practically of less use to the conchologist than might 

 be expected. Not only do all the ramifications of the lobes increase in complexity during 

 the whole life of the Ammonite, a mere notch in the young stage growing into a branch 

 of its lobe, but the number of lobes may also increase with age. According to M, 

 D'Orbigny, all Ammonites have only six lobes at the earliest period of their existence ; 

 namely, the dorsal and ventral lobes, and the superior and inferior lateral lobes on each side 

 of the whorl. In some species this number continues the same through life ; but more 

 commonly the lobes increase in number with the growth of the shell, small auxiliary lobes 

 being added on each side near the junction with the inner whorl. The number of auxiliary 

 lobes varies in different species ; but as this depends on the age of the shell, they form a 

 character of little service. Moreover, as the lobes are most branched near the edge of the 

 whorl, many of the minor ramifications are obliterated on the worn cast of an Ammonite. 



The two sides of the shell being usually symmetrical, it is only necessary to figure the 

 outUne of the septa of the back and one side. But there are some species of Ammonites 

 in which the siphuncle, instead of being placed at the middle of the back, is often found 

 more or less on one side ; and when this is the case, the lobes of the septa are also dis- 

 placed to the same extent, so that the tube remains in the middle of the dorsal lobe. 

 Most of those species are syunuetrical when young, and the change takes place at very 

 different periods of growth in different individuals. This curious irregularity is most often 

 seen in A. denarius, Fittoni, splendens, and Lyelli of the Gault, A. Duneani and Callo- 

 viensis of the Kelloway's Rock. The displacement usually consists in shifting the tube to 

 one of the angles of the back ; but specimens of A. Calloviensis are found with the tube 

 placed near the middle of one of the sides. M. D'Orbigny has illustrated this deformity 

 in describing, in the Paleontologie Frangaise, the species liable to it. 



