178 



PALEONTOLOGY 



The Phylloceratidae and Lytoceratidae persisted in the 

 Alpine-Mediterranean region right through the Cretaceous 

 period. The latter family gives rise to some uncoiled 

 forms (Macroscaphites, Ancyloceras, Fig. 41, h, i) in the 

 Lower Cretaceous, but also giving off a branch family, 

 Desmoceratida;, in which the tendency is towards tighter 

 coiling or involution : this family lasts to the Upper 

 Cretaceous. 



In the Neocomian stage of the Lower Cretaceous, the 

 oppelids and perisphinctids persist, and a very charac- 

 teristic form is Olcostephanns (also found in the higher 



FIG. 51. OLCOSTEPHANUS ASTIERIANUS (D'ORBIGNY) 

 NEOCOMIAN. ( x ^.) 



Plain apertural margin, oblique to radial line of ornament. (After 

 d'Orbigny. ) 



Jurassic strata), coronate to planulate, with perisphinctid 

 aspect (Fig. 51), but a different suture-line of the 

 " inverse " type (the lobes from periphery to umbilicus 

 successively ending farther and farther in front of the 

 guide-line). Besides the uncoiled forms already mentioned, 

 Cnoceras and Toxocems (Fig. 41, /) are common, or more 

 correctly criocones and toxocones, for the generic names 

 as generally used denote groups of homo2omorphs rather 

 than true genera. 



The Barremian stage is specially characterized by the 

 Pulchellida, rather compressed involute forms with ribs 

 of peculiar shape broadening from umbilicus to peri- 



