LIFE IN SECONDARY TIMES 251 



greater — certain Ammonites approached a metre in diameter. 

 They evidently kept either close to the surface or at moderate 

 depths, and we thus understand how it was possible for the 

 transformations to come about that they underwent during the 

 Cretaceous Period. The last convolution of the shell is at first 

 detached from the others as if it hung in the water below the 

 remainder of the shell and served as a ludion ; then it described 

 an upward C-shaped curve, as though the creature's mouth, 

 at first directed downwards, were subsequently upturned. 

 This change in the orientation of the mouth is perhaps only 

 apparent ; we may, in fact, admit that the shell, having 

 originally opened upwards as in other Ammonites, was now 

 oriented in such a way as to keep the mouth as far as possible 

 in the same direction, whenever there occurred any displacement 

 of the centre of gravity, due to the growth of the Mollusc and 

 of the air-filled chambers of the shell, such as would upset the 

 balance struck between the animal and its shell. Having 

 once begun, this uncoiling continues, and ends in the complete 

 unwinding of the shell from its point of formation. Thus we 

 pass from the type of Scaphiles and Crioceras to that of Pictetia. 

 The shell of the last is C-shaped, and the upper hook is coiled 

 spirally. In Hamites the convolutions are no longer spiral, but 

 formed of parts bent at right angles one over the other. Finally, 

 in B acuities the straight portion is so long in proportion to 

 the coiled part that we might think it a reversion to the 

 Orthoceratidae. The actual organization of the Ammonite, 

 moreover, undergoes a kind of degeneration. The folds of the 

 mantle, which follow the sinuosities of the lines of suture, 

 become so simplified as to resemble Ceratites of the Trias, 

 or even Goniatites of the Primary. It was thus, no doubt, 

 that the transition was gradually effected to the form of the 

 little Spirulae of the present epoch, with its interior unwound 

 shell and simple septa. 



The Turrilites, which are twisted corkscrew fashion instead 

 of being coiled in a flat spiral, and which are consequently dis- 

 symmetrical, could not have been produced if the organism 

 had remained either floating or swimming in some homogeneous 

 environment like water. The torsion the shells have undergone 

 must be due to the same causes as those which affected the 

 Gasteropoda, and indicates the existence of a group of crawling 

 Ammonites. 



