io8 EVOLUTION IN THE PAST 



OCTOPI The decline of the belemnites may have been in part 

 caused by the rise of allied and more active creatures, known 

 as octopuses. These voracious molluscs now appearing 

 had hardly any shelly substance to carry. A handicap to 

 elasticity of movement inherited from remote sluggish 

 ancestors had, therefore, been practically removed (Pales- 

 octopus). 



NAUTILI The decay in ammonite and belemnite ranks was in part 

 counterbalanced by the success of cephalopods of more 

 ancient lineage. Some exceptionally large forms of nautiluses 

 now made their appearance. The old blood was reasserting 

 itself. 



UNIVALVED Marine gastropods or sea-snails do not seem to have been 

 MOLLUSCS much affected by the changes going on around. Those with 

 the improved breathing apparatus, initiated in the Trias, 

 and largely in evidence in the Jurassic, were greatly on the 

 increase. But old-fashioned siphonless forms that carried 

 on respiration simply through the skin, or by means of gills 

 and slightly modified mantle-edges, were probably still in a 

 large majority. 



BIVALVED Bivalved molluscs had at no time proved unequal to the 

 MOLLUSCS struggles of life ; nor did the Cretaceous Period find them 

 otherwise. 



There is nothing notable to report about the mussels, 

 scallops, cockles, and oysters ; except that among the last 

 were some exceptionally large forms. Several present-day 

 families of bivalves with burrowing habits first become known 

 in this Period (Mactridce, Saxicavidce, etc.). Other bivalves 

 were following out experiments made in the Jurassic ; and 

 they met with temporary success. The animals appear to 

 have been related to clams ; but they were very unlike clams 

 now living. Their shells had assumed a horn-like shape ; 

 and some rather later forms resided in long, funnel-shaped 

 shells, protected at the top by a lid (Hippurites). These well- 

 accommodated animals became very numerous and widely 

 distributed. But the ostentatious shelly residence was not 

 really significant of great wealth within. The creatures 

 certainly kept up appearances for a long time ; but to- 

 wards the close of the Cretaceous they failed to meet the 



