THE LAMELLIBRANCHIA 81 



in early forms, the fixed valve later becoming conical or 

 cylindrical, the free valve flattened and lid-like. Teeth 



of normal forms . -.'-' 3- of inverse forms . ^ , 



AII.-2 ' AII.-2, PlI. 



according to Prof. Douville, An. and 2 being united 

 into one tooth in each case. The ligament becomes 

 deeply internal and finally disappears, the valves being 

 no longer hinged but the free valve sliding up and down. 

 The whole shell gradually loses all resemblance to ordinary 

 lamellibranchs. Upper Jurassic and Cretaceous. 



In the Upper Jurassic of North-East France is found 

 Diceras (Fig. 23,7), having both valves twisted into a loose 

 spiral, and fixing itself indifferently by either valve. This 

 is the forerunner of a great series of the most remarkably 

 modified of all lamellibranchs. In the Lower Cretaceous 

 there are genera fixed some by the left valve, the majority 

 by the right: in either case the fixed valve tends to become 

 conical or cylindrical in shape, while the free valve takes 

 on the character of a lid or operculum. The genus 

 Requienia still has both valves spirally coiled, but the right 

 valve is quite flat, so that the whole shell has a strange 

 resemblance to a gastropod with a spiral operculum. In 

 the Upper Cretaceous the forms fixed by the right valve 

 develop into still more extraordinary forms, mimicking the 

 rugose corals of the Palaeozoic. Such is Hippurites, the 

 right valve of which forms a cylinder a foot or more 

 in height, of which the animal only occupies the upper- 

 most portion, the lower part of the cavity having been 

 cut off, as growth proceeded, by a series of calcareous 

 partitions (like the tabulae of a coral or the septa of 

 a cephalopod). The left valve fits on as a lid, and 

 the hinge-teeth and adductor muscles are strangely 

 modified. These hippurites grew in reefs, like corals, 

 and they (as well as their Lower Cretaceous prede- 

 cessors) are almost restricted to tropical and sub-tropical 



6 



