CEPHALOPODA. 105 



The Arietes pass by many intermediate forms into the Falci- 

 feri (e.g., A. serpentinus), also characteristic of the upper lias, 

 and these are represented by a few quoit-shaped species 

 (Disci), with sharp backs, in the oolites. 



Ammonites with serrated keels (Amalthei), exemplified by 

 A. spinahis and margaritatus, abound in the middle and 

 upper lias, and again in the oolites (e.g., A. cordatus and 

 excavatus). They are succeeded by the Rothomagenses in the 

 chalk — thick Ammonites with a line of tubercles in the place 

 of the keel. 



Ammonites with channelled backs (Colliciati) are repre- 

 sented in the lias (A. anguliferus), inferior oolites (A. ParJcin- 

 soni), and middle oolite (A. anceps), and in the cretaceous 

 strata by numerous species (e.g., A. serratus, lautus, and fal- 

 catus), remarkable for their elegance. 



Of the species with backs more or less squared, armatus 

 and capricornus occur in the lias, athleta and perarmatus in 

 the Oxfordian. But the oolitic forms which have the back 

 square, and ornamented with two rows of spines when young, 

 like Goweri, Duncani (fig. 81, z), and Jason, become rounded 

 and unarmed in their old age. 



Eound-backed Ammonites abound in the lias and oolites. 

 The snake-like annulatus, the spine-bearing coronatus, and 

 fimbriatus with its ornamented fringes, have been regarded as 

 types of small groups. A more important division (Ligati) 

 is distinguished by nearly smooth whorls, constrictions re- 

 curring at regular intervals. These are seen in A. tatricus, 

 and others related to Heterophyllus ; in many neacomian 

 Ammonites, and in A. planulahts of the lower chalk. 



These constrictions, often accompanied by a prominent 

 rib, undoubtedly indicate periods of rest, when the Ammonite 

 ceased for a while to grow. They may be traced in species 

 belonging to other groups, as well, e.g., in hiplex and triplicatus, 

 as in the Ligati; but most frequently all indications are 



