VARIATION IN TIME 165 



tuberculous forms, such as the Vivipara Sturi and 

 Hornesi, sufficiently different in aspect to the true 

 Vivipara for some malacologists to separate them 

 from these last under the generic name of Tulotoma. 



In the course of the geological ages a few groups 

 can be named, the different parallel branches of which 

 have appeared and rapidly disappeared. The 

 enormous Brachiopod Stringocephalus is only known 

 from the upper Silurian to the middle Devonian, 

 and the strange Uncites is even limited to one single 

 zone of this last rock. The bivalves of the 

 Cardiola group also only lived during the Silurian 

 and the Devonian. Most of the branches of the 

 great family of the Rudistce, or Chamidse have a 

 very limited duration : the Dicerata are stationed 

 in the higher Jurassic, the Requienias in the Ur- 

 gonian, the Hippuritse in the Chalk beginning with 

 the upper Turonian. Among the Pteropods, the 

 curious family of Tentaculites has only lived from 

 the Silurian to the Devonian. Among the Gastro- 

 pods, the Lychni are quartered in the Danian. 

 Many branches of Nautilids appear and vanish in the 

 Silurian epoch alone. Among the Ammonians the 

 isolated group of the Clymenias completes its evolu- 

 tion in the upper Devonian. Finally, most of the 

 families of Trilobites have a very brief evolution : 

 the Conocoryphids in the Cambrian, the Olenids 

 and the Agnostids in the Cambrian and the Ordo- 

 vician, and the Asaphids and the Trinucleidse in 

 the Silurian only. 



In a pretty general way it may be said that 

 the rapidity of evolution of a group is in inverse 

 ratio to its longevity. The phyletic branches which 



