254 GEOGRAPHICAL DISTRIBUTION. 



point out the specific uses of each minute variation, in the 

 arrangement of parts fundamentally the same. 



The geographical distribution of Ammonites in the an- 

 cient world, seems to have partaken of that universality, 

 we find so common in the animals and vegetables of a 

 former condition of our globe, and which differs so remark- 

 ably from the varied distribution that prevails among exist- 

 ing forms of organic life. We find, the same genera, and, 

 in a few cases, the same species of Ammonites, in strata, 

 apparently of the same age, not only throughout Europe, 

 but also in distant regions of Asia, and of North and South 

 America.* 



Hence we infer that during the Secondary and Transi- 

 tion periods a more general distribution of the same species, 

 than exists at present, prevailed in regions of the world 

 most remotely distant from one another. 



An Ammonite, Hke a Nautilus, is composed of three 

 essential parts: 1st. An external shell, usually of a flat dis- 

 coidal form, and having its surface strengthened and orna- 

 mented with ribs (see PI. 35, and PI. 37.) 2d. A series of 

 internal air-chambers formed by transverse plates, inter- 



* Dr. Gerard has discovered at the elevation of sixteen thousand feet in 

 the Himmalaya Mountains, species of ammonites, e. g. A. Walcoti, and A- 

 communis, identical with those of the Lias at Whitby and Lyme Regis. 

 He lias also found in the same parts of the Himmalaya, several species of 

 Belemnite, with Terebratulse and other bivalves, that occur in the Englisii 

 Oolite ; thereby establishing the existence of the Lias, and Oolite for- 

 mations in that elevated and distant region of the world. He has also 

 collected in the same Mountains, shells of the genera Spirifer, Producta, 

 and Terebratula, which occur in tiie Transition formations of Europe and 

 America. 



The Greensand of New Jersey also contains Ammonites mixed with 



Hamites and Scnphites, as in the greensand of England, and Captain Beechy, 



and Lieutenant Belcher found Ammonites on the coast of Chili in Lat. 36 S. 



in the Cliffs near Conception; a fragment of one of these Ammonites is 



preserved in the Museum of Hasler Hospital at Gosport. 



Mr. Sowerby possesses fossil shells from Brazil resembling those of the 



laferior Oolite of England. 



