284 DISTRIBUTION OF THK MOLLUSOA IN TIME. 





Infra Lias 



Lower Lias. 



Middle Lias. 



Upper Lias. 



Total. 



Brachiopods, 



2 



18 



44 



20 



84 



Lamellibranchs , 



111 



78 



115 



82 



386 



G-astropods, 



74 



36 



78 



66 



254 



Cephalopods, 



9 



63 



65 



111 



248 



" It is only in the Upper Lias that the cephalopods surpass 

 the lamellibranchs. 



" The type Nautilus, so rich and varied during paleozoic 

 times, declines more and more through the Mesozoic beds. The 

 last of the Orthoceras became finally extinguished in the Trias. 

 In the Lias appeared Belemnites, which was perpetuated to the 

 end of the Cretaceous period, and the abundance of which in 

 those seas is as remarkable as that of Orthoceras in palasozoic 

 seas. "The Trias is characterized by a great number of the 

 subdivisions of Ammonites. Thus, we are acquainted, in this 

 formation, with 130 species of Arcestes, and with numerous 

 Didymites, Lobites, Tropites, Clydonites, Ceratites, Trachy- 

 ceras, Pinacoceras, Sageceras. Among the uncoiled Ammonites 

 may be cited Cochloceras and Rhabdoceras. The gastropods 

 are very numerous ; the limited fauna having been studied witli 

 care at Saint-Cassian ; they are : Chemnitzia, Loxonema, Rissoa, 

 Eulima,Trochus, Turbo, Pleurotomaria, Cerithium, Helcion, etc. 

 The lamellibranchs belonged to the genera My ophoria, Cardium, 

 Leda, Nucula, Area, Myoconcha, M3^tilus, Avicula, Posidon- 

 omya, Pecten, Ostrea, Dicerocardium, Halobia, etc. 



" The Jurassic contains some peculiar types of Ammonites : 

 Arietites, ^goceras, Harpoceras, Oppelia, Stephanoceras, 

 Peltoceras, etc. ; some Ammonites with uncoiled whorls : Toxo- 

 ceras, Ancyloceras ; numerous species of Belemnites, and scA^eral 

 other cephalopods approaching the Calamaries : Palseoteuthis, 

 Leptoteuthis, Acanthoteuthis, etc. The characteristic gastropods 

 belonged to the genera Bourguetia (Phasianella), Pleurotomaria, 

 Trochotoma, Pileolus, Rimula, Straparollus, Chemnitzia, 

 Eucyclus, Nerinaia, Alaria, Spinigera, Purpuroidea, Cylindrites, 

 Acteonina. We cite among the lamellibranchs, the genera 

 Panopsea, Pholadomya, Astarte, Opis, Hippopodium, Ceromya, 

 Cardinia, Trigonia, XJnicardium, Lima, Pecten, Ostrea, Gryphsea, 

 Gervillia, Pernostrea, Plicatula, Sowerbya, Cyprina, Isocardia, 

 Pinnigera, Diceras, Hypotrema, etc. 



" In the Cretaceous appeared the sections of Ammonites for 

 which have been proposed the genera Sphenodiscus, Schloen- 

 bachia, Hoplites, Acanthoceras, Stoliezkaia ; Ammonites with 

 vmcoiled whorls became very numerous : Crioceras, Ancyloceras, 

 Baculites, Baculina, Ptychoceras, Hamulina, Scaphites, Toxo- 

 ceras, Heteroceras, Helicocei'as, Turrilites. The exaggerated 

 mutations of this type announce its. near extinction. The 

 Belemnites in the lower Cretaceous take forms not less vmusual 

 (B. polygonalis, Emsrici, dilatatus) ; the genus Belemnitella 



