a neio Claseijication of Ammonites. 369 



same function in the Ammonite. It occm*s constantly in 

 certain genera of Ammonites ; but this is not the case with 

 the auricles ; arj^ the irregularity in the form and presence of 

 these organs proves that they were not destined to the part 

 M. Suess has attributed to them. 



On examining these appendages with care, we see that 

 their length is by no means in inverse proportion to that of 

 the chamber. In the Amalthei, in which the chamber forms 

 from half to two thirds of a turn, the margin scarcely pre- 

 sents a slight lateral process ; in the Planulatiy on the con- 

 trary, in which it is much longer, the auricles are often well 

 developed. Their presence itself is very irregular, even in 

 the same species ; it presents great variations with age : 

 M. Waagen has ascertained that tliey often disappear at 

 a certain age. Moreover, sometimes, of two Ammonites 

 of the same species and the same size, one presents auricles 

 and the other a simple margin. As examples of this, 

 Waagen cites and figures * two A. 02)alinus obtained at 

 Zaskale, in Gallicia. It is probable that these appendages 

 had some other physiological function. The species furnished 

 with an Anaptychus do not present auricles at any period of 

 their existence ; those which have auricles, even if only during 

 their youth, have, on the contrary, a true Aptychus. 



The diiFerences which I have just indicated are not sexual 

 differences. In fact, there have been found, at Solenhofen, 

 amongst the Ammonites which contain Aptychi, as many in- 

 dividuals provided with auricles as destitute of them. Now 

 we have seen that the Aptychus is a distinctive sign of the 

 female Ammonite. Certain shells from this same deposit, in 

 which the line of the ring of attachment is still well marked 

 and which have consequently been submitted since their death 

 only to a slow decomposition in which the soft parts alone 

 have disappeared, are not furnished with Aptychi ; therefore 

 they never had any, and they evidently belonged to males. 

 Now they do not present any difference from the female indi- 

 viduals, except perhaps a little more strongly marked orna- 

 mentation. 



The figures of A. steraspis given by Oppel (Palaont. Mitth. 

 pi. 69) are very instructive on this point. 



The Structure of the Shell. — The shell of the Nautilus is 

 composed of two layers — an external layer formed of an 

 aggregate of cells of different sizes, and the largest of which 

 are those nearest the outside (it forms the most important 

 part of the shell properly so called, and M. Suess has named 



• Palieontographica, 18G0, xvii. pi. 40. figs. 6 & 7. 

 Ann. & Mag. N. Hist. Ser.4. Fo/.xi. 24 



