538 Forvrard Progression in Nautiloidea and Ammonoidea. 



The niodo of progression oi" the Ammonite animal in 

 its shells ninst, ol)viously, have been similar to that just 

 describid for Nautilus. In the case of an Ammonite >vith a 

 comparatively simple septal sutnre, or when the anterior part 

 of each lobe is relatively broad, the process presents but little 

 difficulty; when, howeVer, the septal suture is very compli- 

 cated, and especially when the anterior part of each lobe 

 is considerably contracted, the mode of progression of the 

 animal in its shell at first sight presents some difficulty. An 

 easv and satisfactory explanation is, however, possible. This 

 explanation presupposes the attachment of the animal to its 

 shell by means of shell-muscks and an annulus like the living 

 Nautihts as described in a paper to the Linnean Society 

 (Trans. Linn. Soc. ser. ii. (Zoology), vol. vii. pp. 71-113). 

 '1 he marks left by the anterior edge of the annulus (anterior 

 aponeurotic band) on the inside of the shell indicate that the 

 animal progressed forward at an even rate. 



The digitations visible at the septal sutures when traced 

 towards the centre of each septal surface rapidly die out and 

 give place to a nmeh more simply curved surface. During 

 the formation of a septum the visceral hump of the animal 

 was attached to the shell-wall, not only by the boundary of 

 that part of the hump engaged in the formation of the sep- 

 tum, but also by the shell-muscles and the annulus, the 

 forward growth of the animal in its shell being temporarily 

 suspended. On the completion of the septum and the 

 renewed onwards growth of the shell-muscles and annulus, 

 room would be given to the aninial to contract the posterior 

 portion of the visceral hump inwards, and thus release and 

 withdraw all the fine digitations of the mantle from the 

 crumpled edge of the septum. AVheu the creature had ad- 

 vanced sufficiently far forward in its shell, further progress 

 was temporarily susjjended, the mantle digitations were again 

 j)resscd outwards against the shell-wall, and the next septum 

 formed. 



In some cases, when an adequate interval has not been 

 attained, this results in the backward extensions ( = lobes) 

 being pressed upon the forward extensions ( = saddles) of the 

 preceding septal suture, so that the resultant successive septal- 

 suture lines are crowded on each other to such an extent as 

 to be difficult to follow out. Such a case is well shown in 

 the figure given by Pervinquiere of a specimen o{ Mortuni- 

 cerus prorulum, Coquand (Carte Gcol. Tunisic. — Etudes 

 Paleont. Tunisienne. I. Cephalopodes terr. Second. 1907, 

 p. 238, fig. \)7), and again (p. 2U8, iig. 114) in an example 

 of Acuntltucerus uunialciiae, Coquand. These cases are in all 

 probability i)athological. 



