Miss Agnes Crane — Recent and Fossil Ce^yJialopoda. 493 



tain peaks were not i;plieavecl until a comparatively recent period. 

 Tlie bodjr-chamber of the Ammonite seemed to be so small in pro- 

 portion to that of the Nautilus that Dr. Gray^ was led to conclude 

 that the shell was not inhabited by the animal, but was internal, re- 

 sembling that of the existing SpiruJa. The fact is, the last chamber 

 is rarely preserved in an unbroken condition, there being, of course, 

 no septa to strengthen the walls of this last part of the shell, which is 

 in consequence usually crushed and flattened. But occasionally the 

 interior, discoloured by the decomposition of animal matter, furnishes 

 indisputable evidence that the creature inhabited it during life, and 

 was buried in it after death. ^ The siphuncle is next the keel, and the 

 edges of the septa, where they unite with the shell-wall, are beauti- 

 fully foliated in various patterns peculiar to the respective groups. 

 This foliation is very remarkable in some species, and is visible in 

 all when the outer surface of the shell is removed. As in all these 

 fossil genera, the classification of the Ammonitidce is necessarily based 

 on the characters of the shell alone. There are sixteen type-groups, 

 each of which characterizes a certain horizon, and the presence 

 of a member of a group in any deposit denotes its position 

 in the geological series. In the Liassic formation even species of 

 Ammonites are said to be restricted to a certain horizon, and never 

 to occur out of it ; ^ a knowledge of the limits of these groups is 

 therefore of great service to the seekers for any of the rich 

 industrial products for which that formation is so noted. Thus, 

 although the rich ironstone bands occur only in the zone of Ammonites 

 spinatus, manj'^ useless searches for iron were formerly made in the 

 beds characterized by Ammonites annulatus, the outward appearances 

 of the strata being identical. Again, the familiar Ammonites com- 

 munis is found only in the alum shale, and Ammonites serpentinus in 

 the jet-bearing rocks. The numerous representatives of this most 

 extensive genus were originally grouped in sections or families, a 

 very artificial mode of division. The far more consistent method, 

 first inaugurated by Continental palceontologists, of assigning generic 

 names to the various series of characteristic species, is now, however, 

 becoming generally adopted in England. The interesting family of 

 the Ammonitidce reached its maximum in the Jurassic epoch ; it was, 

 however, numerously represented in the Cretaceous seas, and some 

 specimens attained large size, measuring over three feet in diameter. 

 On their total extinction at the close of the Chalk period, they can by 

 110 means be regarded as an effete race, for several varied types, such 

 as Turrilites, Scaphites, and Hamites, etc., made their appearance in 

 the Cretaceous epoch, but, enjoying a very restricted range, died out 



1 Post-Eocene Period. 



2 Ann. and Mag. Natural History, 1845, vol. xv. pp. 257 and 444. 



3 Specimens of A. heterophi/Uus and of A. perarmatus, and many other species, 

 preserved in the British Museum, show the hody-chamher to have been of very large 

 extent.— H. W. 



^ This conclusion, however, so contrary to all known zoological laws regulating 

 the distribution of life in time and space, may well be received with great caution, 

 even when given on high authority and apparently borne out by many and wide- 

 spread facts .-^H. "W. • . , 



