BIOLOGICAL PALAEONTOLOGY 103 



chambered " Bactriticones " of the Palaeozoic ancestors 

 of the class were of such a nature that considerable 

 increase of size, or exposure to rough treatment, could 

 not fail to result in fracture. Although the septa served 

 in some degree as reinforcements to the hollow tubes, 

 they could not, while remaining smooth and distant, 

 prevent rupture across the cavities of the intervening 

 chambers. Since increase of length of the shell was 

 apparently a pre-determined necessity, mechanical 

 efficiency could only be attained by profound modifica- 

 tion of the Bactriticone. A coiled tube is less easily 

 broken than a straight one ; hence a progressive 

 tendency to enrolment and involution was initiated. 

 In this way tubular shells which, if straightened out, 

 would be of unwieldy length and corresponding fragility, 

 came to be accommodated in compact and mutually 

 supporting whorls. Complication of the septa, leading 

 to the production of increasingly elaborate suture-lines, 

 added to the strength due to enrolment, since it became 

 impossible to make a straight fracture across the tube 

 without intersecting some part of an internal, corrugated 

 lamina of shell. The two lines of specialization, accom- 

 panied by more or less irregular elaboration of super- 

 ficial ornament, were progressively followed, in the phase 

 of anagenesis of the class, from the Devonian to the 

 Trias; in the latter period the Ammonites attained 

 their acmaic stage, excelling in structural perfection, 

 specific differentiation and world-wide distribution. 

 But from the Triassic period onwards, one group after 

 another showed signs of failure (the phase of catagenesis), 

 and ultimately became extinct. The expression of 

 degeneracy varied in different groups. Some, repre- 

 sented by such genera as Rhabdoceras, Ancyloceras, 

 Hamites and Scaphites (PI. xiv. fig. 8), retained, and 

 even increased, the complexity of their septal corruga- 



