SIPHONIDA. 493 



(Woodward). The species of Diceras are exclusively con- 

 fined to the Middle Oolites. In this formation in the Alps 

 they occur in such abundance 

 as to give rise to the name of 

 " Calcaire a Dicerates," applied 

 to beds of the same age as the 

 Coral Eag of Britain. 



The genus Requienia is ex- 

 clusively confined to the Cre- 

 taceous period, and differs from 

 Diceras chiefly in having a very 

 inequi valve shell, always at- ^. m.-mcatina. Middle 

 tached by its left valve. The 

 attached valve is the largest, and is spiral, whilst the free 

 valve is small and sub-spiral. 



FAM. 8. HIPPURITIDCE (Rudistes of Lamarck). *" Shell in- 

 equi valve, unsymmetrical, thick, attached by the right umbo; 

 umbones frequently camerated ; structure and sculpturing of 

 the valves dissimilar ; ligament internal ; hinge-teeth 1-2 ; 

 adductor impressions two, large, those of the left valve on 

 prominent apophyses ; pallial line simple, sub-marginal." 

 (Woodward.) 



The Hippuritidce are not only entirely extinct, but are 

 exclusively confined to the Cretaceous rocks, whence more 

 than one hundred species have been described. All the 

 members of this family were attached, and lived in beds like 

 oysters. The two valves of the shell are always altogether 

 unlike in sculpturing, appearance, shape, and size ; and the 

 cast of the interior of the shell is often extremely different 

 to the form of the shell itself. About a hundred species of 

 the family are known, all of which are Cretaceous, occurring 

 in Britain, Southern Europe, the West Indies, North America, 

 Algeria, and Egypt. Species of this family occur in such 

 numbers in certain compact marbles in the South of Europe, 

 of the age of the Lower Chalk, as to have given origin to 

 the name of " Hippurite Limestones " applied to these strata. 



The Hippuritidce have been especially studied by Dr S. 

 P. Woodward, who makes the following remarks upon their 

 structure and affinities : " They are the most problematical 



