i;o INVERTEBRATE PALAEONTOLOGY 



the shell-cavity. It is quite restricted to the Devonian 

 (where, with its allies, it occurs in abundance), and 

 probably represents an unsuccessful neanic experiment 

 in the early evolution of the order. Typical " Goniatites," 

 such as Anarcestes, Tornoceras^ Aganides and Gephyro- 

 ceras, are not uncommon in the Devonian, and often 

 show strong involution and pronounced septal corruga- 

 tion. The last-named genus is interesting, since it was 

 probably directly ancestral to the familiar Ceratites 

 stock of the Trias. The commonest Goniatites of 

 the Carboniferous are Prolecanites and Glyphioceras 

 (PI. xii. fig. 6) (Lower and Upper Carboniferous Lime- 

 stone respectively) and Gastrioceras (Coal Measures). 

 In some parts of the British Carboniferous sequence the 

 two last genera are almost rock-formers, Gastrioceras 

 being particularly abundant in marine bands of the 

 Lower Coal Measures, and supplying much of the cal- 

 careous matter that caused development of " bullions " 

 and " coal-balls " in such seams. In the Permian, 

 Medlicottia continued and amplified the qualities shown 

 in Prolecanites ; while Popanoceras represented a similar 

 post-Goniatite stage, with complete involution and 

 practically " Ammonite " sutures, in a different lineage. 

 No Ammonoids have been found in the Magnesian 

 Limestone; the unhealthy waters of the enclosed gulf 

 in which that deposit was formed were evidently repug- 

 nant to robust types. No forms referable to Dibranchiate 

 Cephalopods have yet been discovered in the Palaeozoic, 

 although Belemnoids (probably modified from some Ortho- 

 ceratid ancestor) were definitely established in the Trias. 



(K) ARTHROPODA 



The two Arthropod groups most characteristic of the 

 Lower Palaeozoic (Trilobita and Eurypterida) persisted 

 throughout the later periods, but showed rapid diminu- 



