AMMnN! 



\MMuNI 



of the Motion, e, f, g, *, ', to the very nucleus of the shell. The waved 

 transverw line* represent the partitions of the chambers. 



The Urge proportion of the outer chamber i very strongly marked 

 in specimens of A mmonittt rottrattu, that have the aperture perfect or 

 Dearly so. 



" Moreover," continues Dr. Buckland, "the margin of the mature 

 Ammonite in in some species reflected in a kind of scroll, like the 

 thickened margin of the shell of the garden snail" (bourrrlct of the 

 French), "giving to this part a strength which would apparently be 

 needlees to an internal shell The presence of spines also in certain 

 species (as in Ammanilet nrmatiu. A. Soxerbii) affords a strong argument 

 against the theory of their having been internal shells. These spines, 

 which have an obvious use for protection, if placed externally, would 

 seem to have been useless, and perhaps noxious, in an internal position, 

 and are without example in any internal structure with which we are 

 acquainted." 



Sir Henry de la Beche has proved from the mineral condition of the 

 outer chamber of Ammonites from the Lias at Lyme Regis, that the 

 entire body was contained in it, these animals having been suddenly 

 destroyed, and buried in the earthy sediment of which the Lias is 

 composed, before their bodies had either undergone decay or been 

 devoured by the then existing crustaceans. 



Dr. Buckland very happily illustrates the different arrangements by 

 means of which a union of lightness and strength is secured to the shell, 

 both from the external conformation and the mode in which the trans- 

 verse plates are disposed ; and as our limits will not allow us to enter 

 minutely into the subject, we must refer the reader to the ' Bridgewater 

 Treatise' for the interesting details, which show that a more perfect 

 instrument for affording universal resistance to external pressure an 

 instrument in which the greatest possible degree of lightness combined 

 with the greatest strength was required could scarcely be imagined ; 

 and must confine ourselves to the doctor's summary : "As the animal 

 increased in bulk, and advanced along the outer chamber of the shell, 

 the spaces left behind it were successively converted into air-chambers, 

 simultaneously increasing the power of the float. This float being regu- 

 lated by a pipe pausing through the whole series of the chambers " (see 

 the cut of Aaunonita obttuut), " formed a hydraulic instrument of 

 extraordinary delicacy, by which the animal could at pleasure control 

 its ascent to the surface or descent to the bottom of the sea. To 

 creatures that sometimes floated, a thick and heavy shell would hnve 

 been inapplicable ; and as a thin shell inclosing air would be exposed 

 to various and often intense degrees of pressure at the bottom, we find 

 a series of provisions to afford resistance to such pressure in the 

 mechanical construction both of the external shell and of the 

 internal transverse plates which formed the air-chambers. First, 

 the shell is made up of a tube coiled round iteelf, and externally 

 convex. Secondly, it is fortified by a series of ribs and vaultings 

 disposed in the form of arches and domes on the convex surface of 

 this tube, and still further adding to its strength. Thirdly, the trans- 

 verse plates that form the air-chambers supply also a continuous 

 succession of supports, extending their ramifications, with many 

 mechanical advantages, beneath those portions of the shell which, 

 being weakest, were most in need of them." 



Ammonitn with perfect mouths. 



Reinecke,* Von Buch,t Zieten.J anil DC Haan are among those 



Mri protogasi Nantllo* ct Argonralw, vnl(to Oarnua Ammonit, in agro 

 Coburgico et rlclno rcperiundoc, dewripdt ct delineavit, etc., I). I. C. M. Kci- 

 necke. Coburgl, 181s, 8vo. 



t Uebcr die Ammoniten in den ilu-rrn GeblrfpuSchichtcn. Gcleten In der 

 Akadrmie der Wluwniwhnftpn, am 1 April, 1830. 4to. Hccucil de Planches 

 d* Pelrtflration retnarquablc-, par I li. llrrlin, 1N11, folio. 



J Ilic Vcmtelncrungro \Vlirtomberpi, *r. Stuttgart, 18SO, and following 

 year*, folio. 



{ Specimen Philonophlcum Inaugurate, cxbibenn Monographiam Ammo, 

 niteorum el Oonlatileorum, etc. IsIJ. I.nftduni Batar. 



who have written treatises on this interesting genus, or have illus- 

 trated it 



The species of Ammonites are very numerous, ami although the 

 arrangement of Yon llu.li is at present the best, it in probable that 

 when more is known of the form of the aperture, it will serve as a 



, :.'..- ],.ir M !-. 



illft with perfect mouths. (Om/iiiwrf.) 



