MESOZOIC FAUNAS 193 



(common in the Gault) prepared the way for Baculites 

 (chiefly Chalk Marl and Chalk Rock in Britain), in which 

 the straight shell is superficially like that of Bactrites, 

 though retaining elaborate sutures. Turrilites, common 

 in the Lower Chalk, showed deceptive resemblance to a 

 Gastropod. Cosmoceratidae were represented by Hop- 

 lites (common in the Gault), Acanthoceras (Cenomanian) 

 and unrolled forms such as Crioceras^ Ancyloceras and 

 Scaphites (P.I. xiv. fig. 8). The last genus is sometimes 

 abundant in the Lower Chalk and Chalk Rock. 

 Pachydiscus, at the last-named horizon, brought the 

 Desmoceratidae to a close ; while ScMoenbachia, one of 

 the commonest fossils of the Chalk Marl, represented a 

 family (Prionotropidae) whose members were strangely 

 reminiscent of the Triassic Ceratitidae. At this stage 

 forms with intense involution, but septal simplification, 

 appeared. Indoceras, and especially Tissotia, had sutures 

 almost identical with those of Ceratites. No Ammonites 

 survived the Upper Cretaceous. In the British Chalk 

 their remains are rare except in the lowest zones and 

 the shallow-water facies represented by " rock-bands." 



Dibranchiate Cephalopoda made their first appear- 

 ance in the Trias (Aulacoceras), and the Belemnitidae 

 vie with the Ammonoids in abundance throughout 

 British marine Mesozoic strata. In the Lias especially 

 their guards are so thickly congested as to constitute 

 local rock- formers. Very large species occur in the 

 Oxfordian, small ones are more characteristic of the 

 Upper Oolites and Lower Cretaceous. The well- 

 known Belemnitella (PI. xiv. fig. 9) was one of the latest 

 British members of the family ; it is used as a zonal 

 index in the higher parts of the Chalk. Degenerate 

 forms with reduced guards persisted into the Eocene, 

 but the Belemnitidae were essentially a Mesozoic family. 

 The allied Belemnoteuthidae had a similar, but more re- 

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