V 

 E. S. L. ScJiwarz — Shell- Stni dure of Ammonites. 255 



or that which cleaves the water, were lifted up in readiness to 

 spring, and this appears to have been one of the functions of the 

 shell. 



The peculiar layer interposed between two successive whorls in 

 some Ammonites has been termed the "wrinkled layer" by Hyatt,' as 

 a translation of the " Runzelschicht " of Sandberger- and Quenstedt,^ 

 or the " Conche ridee " of Barrande.* It is homologous with the 

 " black layer " of the Nautilus, which sometimes presents a striated 

 appearance.^ It has been observed in Clymenia {Oxycl//menia) 

 striata, Miinst., and CI. (Oxi/cJymenia) pseudogonites, Sandb.'' In the 

 Goniatites it is also, occasionally, seen,''' as in G. {Gepliyroceras) 

 intumescens, Beyi\, and G. (Tornoceras) retrorsus, v. Buch. In the 

 early true Ammonites, also, it is very fi-equently well preserved, as 

 in Arcestes, Cladiscites, Ptychites, Gymm'tes,^ Protensites,^ etc. ; in the 

 later forms it has not been so often recorded, but when it does occur 

 it does so in a greater degree. In Am. (Ltparoceras) Henleyi, Sow., 

 it forms strong spiral ribs over the ventral wall. In Am. (Amaltheus) 

 margaritattis, Schloth., it reaches its highest development, as was 

 pointed out by Quenstedt,'° and I have specimens from Dundry and 

 Bridport showing it well. In the latter specimen the wrinkled 

 layer arises about the fifth whorl, or at the age when the keel first 

 becomes prominent, and in two more whorls attains the complexity 

 figured in Figs. 4-6. In the more enlarged drawing (Fig. 5) the 

 inner layer of the larger whorl is seen to end abruptly where 

 it reaches the inner whorl, and takes part only in the first three 

 wrinkles. At the fourth, the true wrinkled layer begins, consisting 

 of a brown horny material pleated into steep parallel folds, which 

 often split or bifurcate. In life it probably consisted of pure 

 conchiolin : and as we often find a black material taking the place 

 of conchiolin, as between the prisms of the shell of Turbo pica, the 

 homology between the black layer of the Nautilus and the wrinkled 

 layer is strengthened. Resting upon the apices of the folds is the 

 dorsal inner layer, which is not continuous with the inner layer, 

 such as is found on the free sides of the shell. 



The only use that I can see for a layer so developed is that it 

 served as a spring cushion whereby the shocks which the animal 

 gave to its shell when shooting backward were deadened, the 

 conchiolin being eminently adapted for the purpose. And this view 

 is supported by the fact that in Am. margaritatus it first appears at 

 the same time that the keel does, which was used to divide the 

 water and allow the animal to dart more quickly. 



1 Bull. Mus. Comp. Zoo!., Cambridge, 1871, vol. iii, p. 91. 

 "^ Verstein. d. Rhein. Schichtsyst. in JSTassaii, 1850, p. 58. 



2 Petrefakteukuude Deutsclilands, pt. i, 1846. 

 * Syst. Silur., vol. ii, p. 23, 1867. 



^ PaliEontographica, vol. iv, p. 184. 



« Verhandl. d. Naturhist. Vereins. d. preuss. Elieinlande, 1853, vol. x. 

 ' Keyserling, Reise in das Petschoraland, St. Petersburg, 1846, p. 274. 

 ^ Mojsisovics, Med. Trias Provinz, Abb. geol. Reichsaust., Wien, 1882. 

 9 Hauer, Trias v. Bosnien, Denksch. Akad., Wien, 1892, vol. lix. 

 JO Loc. cit. p. 93, fig. 14^. 



