256 E. E. L. Schwarz — Shell- Structure of Ammonites. 



Under the name " Epidermiden " there have been described 

 markings on the inner surface of the living-chamber of the fossil- 

 chambered Cephalopoda, which are probably homologous with the 

 soft lattice markings on the inside of young Nautili.^ Barrande 

 (loc. cit.) has described them in detail for the fossil Nautili and 

 Goniatites of the Silurian, under the name " stries creuses " (some- 

 times stries d'eraillure; German, Eitzstreifung^), and noticing that 

 they are absent in forms with a contracted mouth, he argues that 

 their function was to secure the animal in its shell. In these forms 

 they consist of very fine close-set striations oblique to the direction 

 of the ornaments of the shell. Similar structures are developed in 

 Ceratites,^ but especially in the Triassic Ammonites, JPtychites, 

 Klipsteinia, etc. In later forms they have not been frequently 

 observed, probably owing to the small attention paid to casts of 

 the interior when specimens with the shell on can be obtained : 

 however, I have noticed them in a large Am. {Microceras) capri- 

 coniis, Schloth., from Chideock. The "stries creuses" were formed 

 probably during the time that the hinder portion of the mantle was 

 secreting the new septum ; for, while that was going on there would 

 be an absolute necessity for the animal to be kept firmly fixed in its 

 new position, and to be able to withstand the pressure of water 

 from without. The muscles of the mantle therefore became tightly 

 stretched, thereby producing little ridges and hillocks around areas 

 of non-compressibility, and a condition of things arose similar to the 

 " goose-flesh " of the human integument, which is caused by con- 

 traction of the skin round the hair papilla. 



In Am. (Deroceras) Dudressieri, d'Orb.,* Am. (Arieiites) obtiisus, 

 Sow., and Am. (Arietites) stellaris, Sow.,"" there appears to be a third, 

 layer deposited on the exterior of the shell in senile forms, and 

 presents an ornamentation which is unique among the vast series 

 of forms. 



The siphuncle in the Ammonites is often beautifully preserved as 

 a horny tube frequently distorted from its normal circular form, 

 Fig. 3. This is equivalent to the inner conchiolin sheath of the 

 Nautilus' siphuncle, and in many cases (Fig. 3d) there is evidence 

 of a sheath surrounding this inner one, but in no case that I have 

 examined does this second one bear spicules as in Nautilus 

 pompilius, L.^ 



1 Sandberger, G., Schrift d. oberhess. Gesells. f. Natur u. Heilkunde, No. yii, 

 p. 79, 1859. 



2 Sandberger, G., Jahrb. d. Vereins. f. JSTaturkunde, Wiesbaden, vii, p. 292, 1851. 



3 Eck, Zeits. deutsch. geol. Gesells., p. 276, 1879. 

 * Wright, Lias Ammonites, Pal. Soc, pi. xxv. 



5 Oppel, Die Juraformation, Jabresh. d. Vereins. f. Naturkunde, Stuttgart, 

 vol. xii, 1855. 



6 Brooks, Proc. Boston Nat. Hist. Soc, vol. xxiii, p. 380. 



