88 PALEONTOLOGY 



with a furrow on each side of the keel, as in the great Ammo- 

 nites called Bucldandi and Coneybearei, mark the lias period ; 

 they are less plentiful in the oolites, and are represented in 

 the greensands by the Cristati, which are keeled, but not 

 furrowed, and develope a " beak," or process, from the keel 

 when adult. The Arictes pass by many intermediate forms 

 into the Falciferi (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. spinatus 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. angidifcrus), inferior oolites (A. Parlin- 

 soni), and middle oolite (A. anccps), 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, arm at us 

 and capricomus occur in the lias, athleta and perarmatius in 

 the Oxfordian. But the oolitic forms which have the back 

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

 like Gowcri, Duncani (fig. 21, 2), and Jason, become rounded 

 and unarmed in their old age. 



Round-backed Ammonites abound in the lias and oolites. 

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

 jimbriatus with its ornamented fringes, have been regarded 

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

 is distinguished by nearly smooth whirls, constrictions recur- 

 ring at regular intervals. These are seen in A. tatricus, and 

 others related to Heterophyllus ; in many neocomian Ammo- 

 nites, and in A. planv.latvs of the lower chalk. 



