456 E. A. L. 8chcarz—The Apfyclms. 



However, if we consider what would happen to an Ammonite 

 when it had died and sunk in the ooze at the sea-bottom, we see 

 that, supposing the animal exogastric like the living Nautilus, and 

 the Aptychus functioning as a lid, the latter would necessarily have 

 to take up its position in the place inside the chamber where we 

 usually find it. For, being exogastric, the outer part of the animal 

 would be occupied by the mantle space underlying the funnel ; 

 while on the inner side, the great muscles of the arms and head 

 would present an impediment to anything entering the shell; so 

 that, when the mud began to push in, the outer border of the 

 operculum would be forced in first, and the whole thing would be 

 turned on its axis, throwing the rough side downwards and out- 

 wards ; and eventually, when the entire animal was decomposed, it 

 would sink to the under surface of the body-chamber with its 

 umboes looking forward, as we usually find it. Hence there is 

 nothing in the fact that the Aptychus is usually found internal in 

 fossils to preclude its having once acted as an operculum. 



2. Though the horny Aptychus of Goniatites, Arietites, etc., 

 might have been dermal in origin, the calcareous varieties most 

 certainly cannot have been formed simply from the surface. But 

 we never find rough bodies, such as the Aptychi belonging to the 

 group Imbricati for instance, internal, unless they are suspended 

 freely in a cavity, as the otoliths of fishes, or give support to other 

 hard parts as in the vertebrate skull ; and this, coupled with the 

 apparent external uses of the Aptychus, drives one to the conclusion 

 that, though primarily internal, it must have reached the surface by 

 degeneration of tissues external to it. What these tissues were, it 

 is impossible to say with certainty. Perhaps it was preformed in 

 cartilage, for that tissue, as von Jhering has shown, sometimes becomes 

 separated into square cells by means of fibres running through it ; 

 or more probably, the mass of muscle which constituted the hood, 

 or conjoined tentacles, became surcharged beneath the sarcolemma 

 with calcium carbonate, as happens in the case of man and hiber- 

 nating carnivores, where it gives rise to gout ; and the enclosed 

 muscle bundle then decaying, left the cavity of the cell empty, 

 or filled with secondary liquids, etc. Possibly, also, as suggested 

 by the last simile, the Aptychus may have been formed at special 

 resting periods during the life-history of the animal, for the organ 

 has no means of growing when once it is fully formed ; and to this 

 cause may be due the small size of the Aptychus in some species, 

 as compared with the normal aperture ; that is, the lid, once formed, 

 was retained for a long time without increment, though the animal 

 itself went on growing as usual. In other cases it would become 

 thrown off when the animal resumed active habits. 



Although the structure of the Aptychus has many times been 

 figured, especially in Meneghini and Bornemann's classical paper,i 

 yet preparations that I have made show a feature that seems to 

 have escaped the notice of other observers, namely, that the cells 

 of the middle layer communicate one with another, and that their 

 1 Atti. Soc. Toscan. di Sci. nat. 1876, vol. ii. 



