﻿108 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 (Palce- 



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 



