466 



THE JURASSIC PERIOD 



of that genus have persisted to the present time and are found in 

 the Australian seas. Diceras and Pholadomya are likewise com- 

 mon genera, and there are very many others. Among the Gas- 

 tropoda the most significant change lies in the importance which 

 the siphon-mouthed shells (Siphonostomata) now for the first time 

 assume ; examples of this group are Nerinia, A/aria, Purpurina, 

 etc. Of the shells with entire mouths (Holostomata) the ancient 



FIG. 151. — Slab of Trigonia clavellata, from the English Jura. 



Palaeozoic genus Pleurotomaria is as abundant as ever, not begin- 

 ning to decline until the Cretaceous period. 



The Cephalopods are at the very height of their culmination, 

 and are present in an astonishing profusion and diversity, filling 

 whole strata with their heaped-up shells. The Nautiloids differ 

 from those of the Trias in their smoother and more involute shells. 

 The Ammonoids do not display so many types of shell structure 

 as we have found in the Trias, and the genera are mostly different 

 from those of the latter period ; but in number of distinct species 

 the Jura much surpasses the other Mesozoic periods. Phylloceras 

 and Lytoceras continue on from the Trias, but the most abundant, 



