L. F. Spath — Notes on Ammonites. 119 



the writer that the internal pressure of air, gas, or aerated fluid in 

 Aramonoids need not have differed from that in Nautiloids of a 

 corresponding type of shell and mode of existence, that it depended 

 on the depth at which the septum was formed, i.e. external pressure, 

 and that the differences in the position of the siphuncle, in the 

 thickness of the shells, and in the methods of attachment, determine 

 the shape of the septum. Mr. Crick 1 has shown that " not only was 

 the Ammonoid animal, like the Nautihis, at least at some periods, 

 attached to its shell by means of the lobes and saddles of the posterior 

 portion of the body — corresponding to those of the edge of the 

 septum of its shell — but it seems . . . that it was further provided 

 with an annulus in addition to shell-muscles, as in the recent 

 Nautilus. It would appear, therefore, as if the provision of an 

 annulus were an absolute necessity to the animal in addition to the 

 shell-muscles, and most probably Dr. Waagen's explanation of its 

 occurrence is the correct one, viz. that the annulus and shell-muscles 

 served not merely to hold the animal to its shell, but formed also an 

 air-tight band around it, fastening the mantle to the shell." Since 

 the muscles could probably easily become detached, as in Nautilus, it 

 may be assumed that the protrusions of the mantle that went into 

 the lobes tended to strengthen their adherence to the septum by 

 progressive backward penetration of their fibres, thus causing the 

 convexity of the septum. In a cylindrical shell like Lytoceras, where 

 the elements radiate from the centre of the septum, the lobes would 

 attach themselves more or less equally all round the shell-wall, and 

 the more complex the septal edge the firmer the attachment. Such 

 progressive complication is shown, e.g. in the first lateral lobe of the 

 Androgynoceras Henleyi-Bechei-nautiliformis series, and on the other 

 hand it has already been suggested that, on the adaptation to a 

 benthonic existence in such forms as Cochloceras or Rhabdoceras, 

 extreme simplification of the suture-line may result ; for in these 

 the need for firm attachment of the animal to its shell, while muscles 

 and annulus shifted forward after completion of a new septum, was 

 probably less great than in an, active swimmer. 



It has already been mentioned that "the tortuous windings of 

 the foliated margin of the transverse partitions . . . strengthened the 

 shell of Ammonites". 2 Buckland 3 examined at great length the 

 " proofs of contrivance and design ". Apart from the " use of giving 



1 "On the Muscular Attachment of the Animal to its Shell in some Foss. 

 Ceph. (Ammonoidea) " : Trans. Linn. Soc, vol. vii, pt. iv, p. 109, 1898. 



2 R. Owen, Lectures on the Compar. Anat. and Physiol, of the Invertebr. 

 Aftim., 1843, p. 331. 



3 " Geology and Mineralogy, etc. " : Bridgwater Treatise VI, vol.i, sect, iii, 

 "Nautilus," pp. 310-32 ; sect, iv, " Ammonites," pp. 333-57 ; also sects, v 

 and vi, pp. 357-60 ; and vol. ii, pp. 58, 59, 69. 



This author (p. 62, vol. ii) also stated that the "course of the transverse 

 plates was beneath the depressed and weakest part of the external shell, 

 avoiding the bosses . . . which from their form were strong". This is not 

 borne out by the specimen of Hoplites auritus, figured by Swinnerton and 

 Trueman (op. cit., pi. iv, fig. 8), and it seems that in general the septal edge is 

 independent of the position of the tubercles, which are also often irregularly 

 spaced. 



