THE CEPHALOPODA 173 



Phylloceratidae are distinguished by their suture-line 

 (Fig. 47, h), in which the saddles are subdivided by little 

 lobes (lobules) into ovoid " cells," constricted posteriorly 

 (phylloids) ; by the general absence of ornament other 

 than striae ; and by the general tendency to complete 

 involution in the shell (Fig. 46, a). 



The Lytoceratidae also have a " phylloidal " suture, 

 but it often becomes so complex that this character 

 is obscured ; long lateral branches to the median internal 

 lobe are characteristic. They tend to an evolute form, 

 though with exceptions, and in the Cretaceous period 

 yield a large proportion of the uncoiled forms (Macvo- 

 scaphites, Ancyloceras, some species of Criocevas}. In 

 both these families constrictions may occur at intervals, 

 and in the Lytoceratidae these are sometimes marked on 

 the exterior of the shell not by depressions, but by con- 

 spicuous elevations (" flares "). 



II. and III. It will be convenient to describe the rest 

 of the Jurassic ammonites in historical order, indicating 

 their taxonomic position by the number II. or III. in 

 parentheses. 



The Jurassic period opens with the Hettangian age, 

 characterized by the first appearance of ammonites in 

 Northern Europe. These belong to the family Psilo- 

 ceratidae (II., Fig. 48, d). The genera vary from nearly 

 smooth (Psilocevas) to strongly costate (Schlotheimia). The 

 latter genus survives to the end of the Sinemurian age, 

 which is mainly characterized by the Arietidae (III.), a 

 family with uninflected sutures, simple straight ribs bend- 

 ing forward on the periphery (Asteroceras, Fig. 39, etc.) 

 and a keel. The whorl-section is usually quadrate, but in 

 the degenerate Oxynoticevas it becomes acutely sagittate : 

 this and similar oxycones persist into the next age. Along- 

 side of the Arietidae in their acme are found not only some 

 survivors (Schlotheimia} from a previous age, but also the 



