THE CEPHALOPODA 179 



phery, so that the spaces between them are parallel-sided 

 and at last narrower than the ribs themselves : the ribs 

 form sharp tubercles on the edge of the flattened periphery. 



In the Aptian and Albian stages the family Hoplitidce 

 is abundant : it is regarded as derived from the peri- 

 sphinctids, though it differs greatly from them both in 

 aspect and suture-line (Fig. 48, g). These ammonites 

 are rather involute, with flattened periphery, and sigmoid 

 (doubly curved) ribs. The later species tend to be strongly 

 bituberculate, with the ribs gathered in to the tubercles. 

 Such species abound in the Lower Gault Clay of 

 Folkestone, but in the Upper Gault they have almost com- 

 pletely disappeared and are replaced by keeled forms 

 practically unknown in the Lower Cretaceous. The 

 uncoiled forms most characteristic of the Gault or 

 Albian stage are Hamites and Turrilites (Fig. 41, e, j). 



An important family beginning in the Lower 

 Cretaceous but extending into the Upper is the Acantho- 

 cemtidce, characterized by broad straight ribs with a 

 tendency to break up into many tubercles. Characteristic 

 of the Upper Cretaceous are the keeled forms of the 

 family Prionotropidcz (Mortonicems, Fig. 52, Schlcenbachia). 

 In warmer latitudes there appear a number of genera 

 known as pseudo-ceratites (Fig. 45, i, j), not a natural 

 family, but agreeing in a degeneration of the suture- 

 line which reverts to something resembling the ceratitic 

 type of the Trias. These forms are known in Southern 

 Europe, Syria, India, North and South Africa, South 

 America, and the Southern United States. The cata- 

 genesis of suture is not accompanied by any loss of 

 coiling: many of them are strongly involute. On the 

 other hand, the uncoiled Cretaceous genera show complex 

 sutures. Scaphites (Fig. 41, gO.and Baculites (Fig. 46, e] 

 are most characteristic of the Cretaceous beds higher than 

 the Gault. 



