THE CEPHALOPODA 177 



or there may be lappets (sometimes long and elaborate 

 in shape) but they never affect the course of the radial 

 lines, though sometimes they leave indications of their 

 former position by markings (parabolic knots) at intervals. 

 Constrictions occur in some genera (Fig. 46, /, /). These 

 richly varied Bajocian and Vesulian stepheoceratids are 

 continued into higher Jurassic stages chiefly as costate 

 planulates known as the Perisphinctidae (Figs. 46, j; 

 48, h ; 49, g) certainly not a true family, as they are 

 polyphyletic, arising from several different coronate 

 stocks. 



The Bathonian age in Northern Europe was marked 

 by an almost total disappearance of ammonites, but in 

 the Callovian a rich fauna reappears, in which four new 

 families appear alongside the persistent oppelids and 

 perisphinctids. These are (i) the Cardioceratidae, pre- 

 sumably allied to Oppelidae : they pass from platycones 

 to sphaerocones with varying degrees of inflation (Quen- 

 stedtocems, Fig. 49, h, Cardioceras), even to very stout 

 cadicones (Cadoceras, Fig. 49, *) ; (2) the Pachyceratidae, 

 coronates ; (3) the Aspidoceratidae, at first square- whorled 

 and evolute, later (in some genera), rounded and involute, 

 characterized by a modification of the coronate or peri- 

 sphinctid suture, in which the external saddle and first 

 lateral lobe become very large, the lower saddles and lobes 

 being compressed into what looks like a single complex 

 saddle ; (4) the Cosmoceratidae, starting as closely-ribbed 

 coronates (Kepplentes) but rapidly becoming more com- 

 pressed and developing a periphery medianly smooth, 

 often with marginal tubercles, while the lateral area 

 becomes richly bi-tuberculate : long lappets are present 

 (Figs. 46, g; 48,/). This last family hardly outlasts the 

 Callovian, and the Cardioceratidae die out in the Kim- 

 meridgian ; the remainder survive to the end of the 



Jurassic, 



12 



