RANGE OF THE AMMONOIDEA 



675 



tlie Haqjoceratidae we find the Desmoceratidae, of which the genera Desmoceras and 

 Silesites especially characterize the Neocomian and Gault, and Pachydiscus the higher 

 stages of the Cretaceous. Of the Stephanoceratidae the genera Perisphinctes and 

 Olcostephanus, which had survived from the Jurassic, are extinguished in the Lowei' 

 Cretaceous. In the place of the Jurassic Cosmoceratidae appear Ho-plites, Douvilleiceras 

 and Acanthoccras. A peculiar retrograde development of the septa, a reversion to the 

 ceratitic stage, is seen in two families of Cretaceous Ammonites, the southern 



