104 PALEONTOLOGY. 



spar, which has more slowly infiltrated through the pores of 

 the shell into the air-chambers, is of a much lighter colour. 

 In the same collection may be seen exemplifications of injury 

 and repair of the shell. In No. 195, Ammonites Goliaihus, 

 from " Oxford clay," a portion of the shell, at the period when 

 it formed the dwelling -chamber, "had been broken away 

 during the lifetime of the animal, and repaired by fresh 

 nacreous material, wanting the ribbed structure of the origin- 

 nally formed shell."* 



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 cannot even be considered sub-generic. The 

 group (called Cassiani) characterising the triassic period, is 

 remarkable for many-lobed and elaborately- foliated sutures 

 — a circumstance more important because it is the oldest 

 group, and associated with Ceratites and the last-surviving 

 Goniatites and Orthocerata. They abound in the "alpine 

 limestone " of St. Cassian, and Halstatt in Austria. A second 

 group (Arietes), having the back keeled, with a furrow on 

 each side of the keel, as in the great Ammonites called Buck- 

 landi and Coneyhearei, mark the lias period ; they are less 

 plentiful in the oolites, and are represented in the greensands 

 by the Cristati, which are keeled, but not furrowed, and 

 develop a "beak," or process, from the keel when adult. 



* 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. 



