AMMONITES — ZOOLOGICAL, ETC. 



41 



very important considerations on the enormous differences 

 which we remark between the internal cast of the shell and 

 the various external appendages. If the internal deposit of 

 matter which thickens the shell within was uniform, it would 

 follow that the extraneous matter deposited therein^ after the 

 death of the animal would give an exact representation on 

 the cast of the external appendages of the shell ; the ribs 

 would there appear as angular, the spines as sahent; but the 

 best proof I can adduce of the inequality of this internal 

 thickening by the animal, is, that such species as have the 

 shell strongly striated, are frequently entirely smooth in the 

 state of a cast, as we see in A, Velledcs, latidorsatus, Mayo- 

 rianus, Duvalianus, Juilleti, Calypso, Guettardi, etc.; the 

 sharp spines which likewise ornament the shells of others 

 disappear entirely in the cast, where they are sometimes re- 

 presented by a faint appearance of a tubercle, as we observe 

 in A. Clementinus, mammillaris, and LalUerianus. It follows 

 from this difference, between the cast and the shell, that the 

 greater or less saliency of the ribs or spines, their presence 

 or absence ought not to authorize the creation of a new spe- 

 cies unless we are enabled to study a great number of indi- 

 viduals in different stages and states, or unless it is autho- 

 rized by very marked characters in the lobes or form of the 

 spiral coil or arch. 



Modifications of the external characters of Ammonites. — 

 These modifications arise from many causes, from the natu- 

 ral variable limits of species or varieties, from accidental cir- 

 cumstances, and from age or sex. I shall treat of these dif- 

 ferent questions under separate heads. 



Natural varieties.— ^^ho, limits of true varieties amongst 

 the Ammonites are more or less extended, according to the 

 species. There are some of which all the individuals pre- 

 sent identically the same external appendages, when they 

 have arrived at the same stage of growth, and this is the ge- 



