CEPHALOPODA 87 



upon the petrifactive processes after death. The liassic clay has 

 penetrated as far as the retracted soft parts of the ammonite 

 permitted : the decomposing mollusk had been partially- 

 replaced by crystals of silex, discoloured by the pigmental or 

 carbonized parts of the animal. The silex, which has more 

 slowly infiltrated through the pores of the shell into the 

 air-chambers, is of a much lighter colour. In the same col- 

 lection may be seen exemplifications of injury and repair of the 

 shell. In No. 195, Ammonites Goliathus, from " Oxford clay," 

 a portion of the shell, at the period when it formed the 

 dwelling-chamber, "had been broken away during the life- 

 time of the animal, and repaired by fresh nacreous material, 

 wanting the ribbed structure of the originally formed shell." 1 * 



The species of Ammonite exceed 500 ; and their range is 

 co-extensive with that of the secondary rocks. They are 

 found throughout Europe, and at the Cape, in Kamtschatka, 

 Thibet, and S. India. They are absent from a large area of 

 the United States, but are found in the cretaceous strata of 

 New Jersey, Missouri, and the West Indian Islands ; also in 

 Chili and Bogota. 



The sections into which, for the sake of convenience, 

 this extremely natural group has been broken up, are very 

 ill-defined, and have no pretension to be considered sub- 

 generic. The group (called Cassiani) characterising the 

 triassic period, is remarkable for many-lobecl and elaborately- 

 foliated sutures — a circumstance more important because 

 it is the oldest group, and associated with Ceratitcs and the 

 last-surviving Goniatites and Orthoccrata. They abound 

 in the "alpine limestone" of St. Cassian, and Hallstatt in 

 Austria. A second group (Arietcs), having the back keeled, 



* Catalogue of Fossil Invertebrata, Mus. College of Surgeons, London, 4to, 

 p. 43, in which work the writer has described upwards of 350 specimens, 

 illustrative of the different sections of Ammonitidae, collected by John Hunter in 

 the last century. 



