LIFE IN SECONDARY TIMES 247 



It was at this time, too, that the hermaphrodite Gasteropods 

 greatly increased— and terrestrial Snails and Slugs, the future 

 freshwater Limneae and Physse, the Bullae, first of the series of 

 marine Molluscs in whom we can follow the gradual loss of the 

 shell, and Acteons, the least modified of the Opisthobranchs, 

 themselves probably the forebears of the open-sea Pteropods, 

 which fly in water by the aid of two large wings dependent 

 from their feet as butterflies fly in the air. 



The bivalve Molluscs were not behind the Gasteropods 

 in progress. During the Jurassic period the majority of the 

 varieties existing to-day were added to those we have already 

 come to know. But there were others as well. We have 

 nothing in our seas that can be compared to Diceras, whose 

 two cow-horned valves, joined at their base, faced each other ; 

 to Requienia, in which only one of the horns persisted, the 

 other being reduced to a simple operculum, closing the orifice 

 of the first ; to the Rudistae, whose large valve exhibits a form 

 and texture so disconcerting that it has suggested the possibility 

 that they are really operculated polyparies like Calceola sandalina 

 of the Devonian strata. They are considered to-day to be 

 related to simple existing bivalves like the Chamidae, which 

 has one of its thick valves attached to rocks, as in the case 

 of Oysters, and the other one free. The powerful hinge 

 uniting the two valves in fact resembles that which unites 

 the two valves of the Rudistae. Among living Lamelli- 

 branchs, the only variety having a valve by which the 

 creature attaches itself, developed in a way recalling the 

 Rudistae, are the ^Etheriidae, found only in the rapids of Central 

 African rivers. Dr. Anthony believes that the exaggerated 

 development of this valve is due to the continuous action 

 of the violent currents to which it has been subjected. He 

 surmises that the Rudistae lived in waters in which they were 

 violently buffeted by the waves. Like the coral reefs they 

 replaced in many localities, they formed a defensive bulwark 

 for the continental masses. 



Throughout the Cretaceous Period the Madrepores were 

 gradually retreating southwards as though the temperature 

 were progressively falling. The polar regions then enjoyed 

 a relatively temperate climate, but the south of 

 France and southern Europe still retained their tropical 



