RANGE OF THE AMMONOIDEA 



675 



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

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

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

 Olcostephanus, which liad survived from the Jurassic, are extinguished in the Lower 

 Cretaceous. In the i)lace of tlie Jurassic Cosmoceratidae appear HopliteSy Douvilleiceras 

 and Acanthoceras. A peculiar retrograde development of the septa, a reyersion to the 

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



