538 HISTORICAL PALEONTOLOGY. 



along with numerous forms of Ostrea itself, and the genera 

 Trigonia, Lima (Plagiostomd), Pholadomya, Cardinia, and 

 Avicula may be mentioned as comprising most of the com- 

 moner forms. One of the most remarkable, however, of the 

 Oolitic genera of Lamellibranchs is Diceras (fig. 189), com- 

 prising certain singular shells allied to the existing Chatna. 

 These are so abundant in a limestone of the Alps of the age 

 of the Coral-rag as to have gained for this formation the name 

 of " Diceras Limestone." 



Of the Gasteropoda there are many examples of the ancient 

 genus Pleurotomaria ; but on the whole the Univalves have a 

 modern aspect. Holostomatous Univalves, such as Nerita, 

 Patella, Natica, Turritella, Chemnitzia, and Nerinaa, still hold 

 a predominant place ; and species of the last-named genus are 

 especially characteristic of parts of the series. The Tertiary 

 and modern genus, Cerithium, also makes its first appearance 

 here. Though the Holostomata still predominate, there is now 

 a fair proportion of the carnivorous siphonostomatous Univalves, 

 and many of these are referable to existing genera. Thus, 

 with the extinct Purpuroidea are found forms belonging to 

 such' genera as Pteroceras, Rostellaria, Buccinum, Fusus, Murex, 

 and Pleurotoma. The Cephalopoda are exceedingly abundant 

 all through the Jurassic series, and are represented by both 

 Dibranchiate and Tetrabranchiate types. The Dibranchiate 

 Belemnitida here attained their maximum of development, 

 many beds being literally charged with the guards of these 

 extinct cuttle-fishes. The Tetrabranchiates are represented 

 by various species of the persistent genus Nautilus, but more 

 especially by species of Ammonites, which are extraordinarily 

 plentiful and of the most varied forms. Speaking generally, 

 Ammonites and Belemnites may be stated to be the charac- 

 teristic fossils of the Jurassic period. 



In the fresh-water strata of the Oolites (Purbeck beds), the 

 Molluscs, as a matter of course, belong to forms which now 

 inhabit fresh water. Thus, amongst the Bivalves we have the 

 genus Cyrena, and amongst the Gasteropods we meet with the 

 genera Planorbis, Physa, Paludina, and Melanopsis. As regards 

 the Vertebrates, little need be said about the Jurassic fishes, 

 which belong to the Ganoidei and Elasmobranchii. The Ganoids 

 now possess, many of them, symmetrical tails, and the most 

 important genera are Tetragonolepis, Dapeditis, sEehmodus, 

 Pycnodus, Leptolepis, and Aspidorhynchus. The Cestraphon 

 are represented by Hybodonts (Hybodus and Strophodus) and 

 Acrodus. Lastly, the true Sharks are not without Jurassic 

 representatives (Notidanus). 



