LAMELLIBRANCH1ATA. 



231 



very inequivalve shell, always attached by its left valve. The 

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

 is small and sub-spiral. 



FAM. 8. HIPPURITID^E (Rudistes of Lamarck). "Shell in- 

 equivalve, 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 Hippuritidcz are not only entirely extinct, but are ex- 

 clusively 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, oc- 

 curring 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 Hippiiritid<z have been 

 especially studied by Dr S. P. 

 Woodward, who makes the fol- 

 lowing remarks upon their struc- 

 ture and affinities : " They are 

 the most problematical of all fos- 

 sils ; there are no recent shells 

 which can be supposed to Belong 

 to the same family ; and the con- 

 dition in which they usually occur Fig . ^._H ippurites Totlcasiana . 



has involved them in greater Ob- A lar S e individual, with two smaller 



scurity. The characters which nes attached to * 



determine their position amongst the ordinary Bivalves are 



the following : 



" i. The shell is composed of two distinct layers. 



