370 M. E. Favre on some Works relating to 



it ostracum), and an internal nacreous layer formed of very 

 small cells, which constitutes the septa and lines the inner sur- 

 face of the ostracum. The former is secreted by the mantle ; 

 the latter by the body of the animal. This same structure has 

 been recognized in many Ammonites, notwithstanding the 

 difficulties which observation presents ; it is observed, in par- 

 ticular, in many Ammonites of the Fimhriati group. 



M. I3arrande has established the fact that in many of the 

 palfBozoic Cephalopoda the organization was such that not 

 only the animal was entirely lodged in the shell, but it could 

 not put out more than a comparatively restricted number of its 

 organs. This character coexists generally with a great thick- 

 ness of the shell, due very probably to the nacreous layer, 

 and with certain swellings of this layer which M. Barrande 

 has named organic deposit. The structure of certain Ammo- 

 nites presents some analogy with this latter fact. In A. cym- 

 hiformis^ of the Trias of Hallstatt, the surface of the ostracum, 

 garnished with the stride of growth of the shell, is seen 

 continuing regularly without interruption to the anterior 

 margin ; and it is only where the shell is broken that we 

 observe on the cast deep periodical fuiTOws, corresponding to 

 so many folds or varices which were formed regularly on the 

 inner surface of the shell, and which occur in the youngest in- 

 dividuals. Generally these varices do not represent former 

 apertures ; for they are not parallel to the lines of growth of 

 the ostracum, as is seen in A. Jarhas. The periodical arrests 

 of growth which are indicated by these varices have nothing in 

 common with those which are necessary for the formation of 

 the septa. The constrictions which are observed in the Planu- 

 lati^ for example in A. polygyratus^ are of a totally different 

 nature ; they do not accord with a varix of the interior of the 

 shell, but they are produced by folds in the ostracum, without 

 any change in the thickness of the shell : M. Suess calls them 

 contractions. The varices and the contractions have this in 

 common, that in each group they are only observed in those 

 individuals which have the aperture but little elevated; in 

 Arcestes they exist in A. cynibiformis^ but they are wanting 

 in those which have an elevated aperture, such as A. Layeri^ 

 Metternichi {Pinacoceras, Mojs.). The varices are only seen 

 in OoniatiteSy Arcestes, Phylloceras, and Clymenia. The con- 

 traction? are seen in Lytoceras, Perisphinctes, and many other 

 Ammonites. The distinction of the contractions and the 

 varices seems to agree with that of the great groups of 

 Ammonites. 



The wrinkled layer [Runzehchicht) is formed by a deposit 

 of calcareous folds in the neiarhbourhood of the mouth a 



