170 G-UIDE TO THE FOSSIL INVEETEBRATE ANIMALS. 



Gallery 

 VII. 

 Wall-ease 

 9. 



Wall-case 

 5. 



Table-cases 

 10, 11, & 12. 



Wall-case 

 5. 



Table-case 

 12. 



Table-cases 

 12 & 13. 



of Holcostcplumus from the Portlandian near Moscow, showing 

 the iridescent shell. The large Holcosteplianus gig as, and the 

 still larger Perispliinctes giganteus, from the Portland Stone 

 of England are in the Wall-case. A small specimen of the 

 latter species had the shell turned into silica, and the infilling 

 limestone has been dissolved out, exposing the shape of the 

 septa and the position of the siphuncle. 



The Lower Cretacous rocks have not furnished many 

 ammonites, but the large " Hoplites" Deshayesi from the 

 Lower Greensand carries on the general plan of the genera 

 just mentioned. Other new genera appear in the series from 

 the Albian of EscragnoUes (Var.), and from the coaeval Gault 

 of Folkestone. The iridescent appearance of these and other 

 ammonites previously noticed is due to the solution of the 

 outer layers of the shell, by which the inner nacreous layers 

 are exposed. Here we meet with Hoplites, characterised by 

 a broad groove on the outer margin of its shell, similar to 

 that previously seen in the otherwise unlike Schlotheimico of 

 the Lower Lias and Parkinsonia of the Middle Oolites. The 

 specimens of this genus from Folkestone form a series 

 illustrating the decline of ornament from tuberculate, 

 through ribbed, to smooth. Similarly Schlocnhachia shows a 

 decline from highly tuberculate to ribbed. S. rostrata marks 

 a wide-spread horizon in the Albian, and there is a splendid 

 series of it from both Gault and Upper Greensand, preserving 

 the long rostrum at the shell-aperture. Phylloceras Guettardi 

 and several species of Holcodiscus show periodical constric- 

 tions of the shell-aperture. The specimens from the Cam- 

 bridge Greensand are derived from the underlying Gault, and 

 those from the lied Chalk seen in the next case are also of 

 that age. 



The Cenomanian forms in the Table-case include 

 the characteristic Acanthoceras rotomagcns from Eouen, 

 Brahmaites from Pondicherry, and Pachydiscus from various 

 European localities. The Upper Cretaceous rocks of South 

 Dakota yield specimens of Placenticeras with elaborate 

 sutures. Tissotia, on the other hand, from the Senonian of 

 Algeria, shows that return to a Ceratite form of suture which 

 is found in many late Cretaceous ammonites. Among the 

 British specimens from the Chalk Marl and Chalk are many 

 figured in D. Sharpe's monograph published by the Palaeonto- 

 graphical Society. Those from the Chalk Marl include a 

 tine series of Schlocnhachia ranging from the tuberculate 

 Schlocnhachia Cou/pei, through the more or less ribbed 



