THE CEPHALOPODA 175 



that of Arietidae in the much deeper first lateral lobe 

 (Figs. 45, /, h; 46, b, c ; 48, b). 



In the Whitbian, the hildoceratids (Harpoceras, Hildo- 

 ceras, Pseudolioceras) are accompanied by planulate dero- 

 ceratids, some tuberculate (Peronoceras, Porpoceras), others 

 degenerated to costate the dactyloids (Dactylioceras). In 

 the Yeovilian and Aalenian, other hildoceratids (Lioceras, 

 Ludwigia)a.re accompanied by the last polymorphids, some 

 closely resembling hildoceratids in aspect (Dtimortieria), 

 others quite distinct (Tmetoceras). 



In the Yeovilian (sporadically), the Aalenian (com- 

 monly), ending in the earliest Bajocian, occur the Ham- 

 matoceratidae (II.), tuberculate with rounded periphery 

 and deroceratid suture-line, which develop a keel and 

 acquire striking resemblance to hildoceratids and 

 sonninines, from which they are distinguished by the 

 suture-line. 



As we pass from the Aalenian to the Bajocian comes 

 the greatest faunal change of the Jurassic : here the line 

 between Lower and Upper Jurassic epochs should be 

 drawn, though the old stratigraphical boundary between 

 Lias and Oolites comes at least an age earlier. The 

 hitherto dominant families die out suddenly, and three 

 new series rapidly establish themselves. First are the 

 Sonnininae (III., Fig. 46, /), a sub-family of the 

 Amaltheidae, though separated from it, so far as present 

 knowledge goes, by nearly three ages. They are 

 characterized in general by a catagenesis of ornament, 

 the inner whorls being spinose, the outer plainly 

 ribbed or even smooth (a marked contrast to the 

 anagenetic ornament of the Domerian amaltheids), but 

 the suture-line becomes more elaborate as the ornament 

 declines. The keel is in many cases a septicarina 

 (p. 147). The Sonnininae are almost confined to the 

 Bajocian ; but the other two new families last to the end 

 of the Jurassic period. 



