i?4 PAL/EONTOLOGY 



primitive ancestor of forms which attain their acme 

 later Microderocems birchi being the first. This is a 

 widely umbilicate planulate form, whose inner whorls 

 show anagenesis from smooth to costate and tuberculate. 

 Xipheroceras (p. 134) is an allied form, later in time. 

 From some such form may have sprung the Derocera- 

 tidae (II.), which attain their acme in the Charmouthian 

 stage forms with rounded whorl-section, costate tuber- 

 culate or spiny, and a highly ornate, much inflected suture 

 (Fig. 48, e). Alongside these are found two other families 

 the Liparoceratidae (III.), which begin as "capricorns " 

 much like Xipheroceras, and rapidly develop into multi- 

 tuberculate unkeeled sphaerocones with uninflected 

 suture-line, and the Polymorphidae (II., Fig. 46, e), with 

 an inflected suture, which also begin rather like capri- 

 corns, but become bi-tuberculate (two tubercles on each 

 side) and develop a keel later on. Here also appear 

 Cctloceras pettos, the first Jurassic coronate type, ancestral 

 to the dominant families of the Upper Jurassic ; and 

 Echioceras, a genus which like the Polymorphidae passes 

 from a planicostan to a keeled stage, and may be ancestral 

 to the Hildoceratidae so important a little later. 



In the Domerian stage, there is an irruption of a new 

 family, Amaltheidae (III.), which have several features 

 in common with the Arietidae, but in which discoidal 

 forms predominate, and the keel has typically a knotted 

 appearance (Figs. 45, e ; 48, a ; and Frontispiece). 



The Whitbian, Yeovilian and Aalenian stages are 

 dominated by the enormously prolific Hildoceratidae (III.), 

 which are already abundant in the Domerian of the 

 Mediterranean region, though rare in Britain. They are 

 characterized by a more or less doubly-curved or sickle- 

 shaped radial line of ornament ; the whorl-shape and 

 degree of involution vary greatly, but there is always a 

 keel, and the suture-line is uninflected, but differs from 



