E. A. L. Schwarz — The Aptychus. 



457 



cavities open on to the outer surface by means of trumpet-shaped 

 apertures (Fig. 3). This shows that there was something in the 

 cells that required taking to the surface, and the suggestion of 

 degenerate muscle supplies a clue to the meaning. The muscle 

 would decompose slowly, having only a limited communication 

 with the exterior, and gases would naturally force their way 

 through the substance ; but as the calcite of the Aptychus is 

 peculiarly compact, it would have to make definite conduits for 

 itself. This gas would probably be poisonous or evil smelling, so 

 that, when the animal was resting during the elaboration of eggs, 

 and the Aptychus applied to the opening, a natural defence would 

 be secured against other predacious Mollusca, worms, and especially 



WQ 



a 



'AV<^. 







Fig. 3. Section of Apty chics Icevis, von Meyer, cut at right angles to the hinge- 

 line in about the centre, a, a, a, communicating cells of the middle layer 

 opening by two orifices on the exterior d, d; b, b, ligament pits ? of the 

 hinge -Uue ; c, secondary outer layer of calcareous material ; e, e, inner 

 layer. 



boring algee, which usually manage to secure a foot-hold on resting 

 shells, and which, as far as I have seen, the Aptychus is free from, 

 though the pearly shells which contain them nearly always show 

 abundant evidence of their ravages. 



It is in this connection that I think we should look for the reason 

 why the Aptychus is so frequently preserved, while no trace of the 

 Ammonite shell is discoverable in the same beds. Fuchs ^ advanced 

 the explanation that the shells consist of aragonite, and the Aptychus 

 of calcite ; but this difference would not sufficiently account for the 

 phenomenon where the action of solvent waters went on for a great 

 length of time. Also, although Sorby - and others quote the specific 

 gravity of the Nautilus shell as 2-9 (aragonite), I have tried it with 

 every possible care, both in the specific-gravity bottle, and the Joly- 

 spring balance, and have found it 2-68 (calcite) ; the structure and 

 properties of the Ammonite shell are so exactly similar to that of 

 the recent Nautilus, that what obtains for the one holds good for 



1 Sitz. ber. d. k. Ak. d. Wiss., Math.-nat. cl. Bd. Ixxvi. 1877, p. 329 ; also, 

 Yerhandl. d. k.k. geol. Eeichsanst., Wieu, 1879, no. 9, p. 186. 



2 Presidential Address, Geol. Soc, 1879, p. 30. 



