a new Classification of Ammonites. 365 



Buck was the first to suppose that they belonged to tlie Am- 

 monites, and that eacli species has a determinate form of 

 aptychus. Oppel (Paliiont. Mittheil.) demonstrated this fact, 

 and ascertained that they have always a, perfectly definite 

 position in the neighbourhood of the siphonal side of the 

 last chamber when the fossil is in a normal state of pre- 

 servation *. 



Three kinds of Aptychus have been distinguished : — Apty- 

 chiis properly so called; AnaptychuSjV7\\\c\\ is characteristic 

 of the groups of the Arietes and AniaUIiei'j and Sidetes, of 

 which the Ammonite is not yet known, and which belongs to 

 the Cretaceous formation. 



The form of the Aptychus is generally known. The shell 

 consists of three layers of different textures, of which the two 

 external ones are often detached. The inner layer is thin, 

 homogeneous, and often impregnated with organic substance ; 

 it is marked with fine lines of growth, and sometimes also 

 with radiating lines. The middle layer, which is the thickest, 

 is distinguished by its structure of juxtaposed canals. 



The outer layer disappears easily ; it has not always been 

 observed. In the thick Aptychi of the Perarmati [A. cellu- 

 losi) it is very thin and pierced with very small holes ; in the 

 Aptychi of the Flexuosi and Falciferi [A. imhricati) it forms 

 a thin homogeneous layer, destitute of pores, which often be- 

 comes detached ; in the Aptychi of the Planulati it is covered 

 with small points. It is particularly developed in the Aptychi 

 of the Alpine strata; in many of them (A. jmnctati, Zitt.) the 

 surface of the thick middle layer is, as in the Imhricati, gar- 

 nished with imbricated folds. But while in these last the 

 outer layer is very thin, it is thickened in the others so as to 

 fill up the intervals of the projecting folds, so that well-pre- 

 served specimens seem nearly smooth ; their surface is covered 

 with round pores, which are sometimes pretty large, arranged 

 in a radiating order, each row corresponding to a furrow of the 

 middle layer. A. profundus, Pict., alpinus, Giimb., striato- 

 punctatus, Voltz, cuneiforviis, Oost., radians, Coq., and Mal- 

 bosi, Pict., present this structure. We do not yet know to 

 what group of Ammonites they correspond ; for they are very 

 abundant in certain beds of the Alps, in which Ammonites are 

 scarcely ever found. This fact, which has repeatedly fur- 

 nished an argument against the opinion that the Aptychus is 

 an integral part of the Ammonite, may be explained in various 

 ways. We may suppose that after the death of the animal 



* M. Schluter has ascertained that the aptychus of the Scaphites occu- 

 pied exactly the same position (Cephal. der ober. deutsch. Kreide, 1872, 

 pi. 25. figs. 5 & 6). 



