364 M. E. Favre <?« some Works relating to 



of the shell, and often beyond this edge on a pedunculated 

 organ, exposed to all sorts of external dangers. The anatomy 

 oi the Nautilus, in conjunction with observations made directly 

 on well-preserved Ammonites, lead him to a very different 

 result from that obtained by M. Suess. This I shall explain 

 hereafter. 



It has long been a question how the animal of the Ammo- 

 nite advanced in its shell, and how it formed its septa. The 

 mode of progression was evidently the same as in the Nautilus. 

 The researches of M. Keferstein, and those of M. Waagen, on 

 the anatomy of the latter animal seem to have settled the 

 question. The animal grows periodically; at certain moments, 

 which are for it a time of repose, it remains fixed : the poste- 

 rior part of its body, which is free, secretes calcareous matter 

 and forms the septum ; at other times this part secretes air, 

 and the animal advances slowly. All its periphery is bound 

 to the shell by a thin layer of conchioline*, of which the outer 

 margin has the form of a ring (annulus), marked in the inte- 

 rior of the shell by a band from 1 to 2 millimetres in breadth. 

 The adductor muscles are attached by a thicker coat of the 

 same substance ; the marks which they leave on the shell in 

 the last chamber have a perfectly definite form. The whole 

 animal, the posterior part excepted, is therefore united to the 

 shell, and the chamber is hermetically closed. 



This explains how the air can accumulate, how the animal 

 can resist variations in the pressure of the air according as it 

 is at a greater or less depth, and also how the soft parts thus 

 sustained could, in the Ammonites, secrete the delicate lobes 

 of the septa always in the same position and on the same spiral 

 line. The mantle extends in front of this attaching ring [Haft- 

 ring) ; it is composed of two parts — one, which is very short, 

 corresponding to the antisiphonal region of the animal ; the 

 other, which is much longer, corresponds to the siphonal re- 

 gion, and secretes the shell with which it is connected by its 

 outer margin. Contrary to the opinion of M. Suess, the form 

 of the margins of the aperture has no direct relation with the 

 position of the adductor muscles ; it depends entirely on the 

 form of the mantle. 



Aptgchus. — The most various opinions have been put for- 

 Avard as to the nature and functions of the Aptgchus f. L. von 



* A substance resembling epidermose and containing about C 50, H 6, 

 and N 16-5 per cent. 



t M. Coquand published, in 1841, ' Considerations sur les Aptychus,' in 

 which he sums up all the opinions brought forward up to that date as to 

 the nature of these singular organisms. He endecivours to demonstrate 

 that these shells belonged to an extinct family of naked Cephalpooda. 



