166 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery have descended from totally different ancestors, and they may 

 thus be placed in distinct genera, perhaps in distinct families. 

 Unfortunately this method of study has not yet been pursued 

 long enough for investigators to have settled the numerous 

 problems presented by the very large numbers of these 

 fossils ; nor are the solutions that have hitherto been pub- 

 lished always found to agree. Hence the classification and 

 nomenclature of the ammonites has for some years been in a 

 state of transition, to the great perplexity of geologists who 

 wish to use these widely distributed fossils for the identifica- 

 tion of various strata and to quote names long familiar in that 

 connection but now requiring emendation. It is impossible 

 to alter the arrangement and naming of a great exhibited 

 series like that of the British Museum to accord with a 

 rapidly advancing classification. Those important questions 

 must be studied in original memoirs, and these pages can 

 only mention a few of the more conspicuous specimens. 



The smaller ammonites in the Table-cases are arranged 

 for the most part under the geological Ages, the foreign 

 specimens of each Age being placed in the same Case 

 as the British ones. The larger specimens in the Wall- 

 cases begin with Liassic forms in Cases 12 and 11, pass to 

 Bajocian in 10, and Oxfordian in 9 ; then, crossing the 

 Gallery, continue with the uppermost Jurassic in Case 6, 

 Lower Cretaceous in 5, and Upper Cretaceous species in 

 4 and 3. We begin with the oldest. 

 Table-case From the Himalayan Trias are 



shown Ptychites, Carnites, and Gym- 

 nites, smooth shells, with sutures 

 not far removed from those of 

 goniatites. Among many specimens 

 from the Upper Trias of Hallstadt 

 in Austria, Monophyllites and Bha- 

 copliyllites, which have primitive 

 sutures with leaf-like saddles, start 

 the line of Phylloceratidae. In 

 Finacoceras Metier niclii, on the other 

 hand, the sutures have already ac- 



quired an extraordinary complexity, 



Fig. U.-Ceratites nodosus ^est shown in some large speci- 

 from the Muschelkaik. mens and a Section in Wall-case 12. 



Sutures of rather simpler type are 

 clearly shown in specimens of Cladiscites multilobakis. From 

 St. Cassian in the Tyrol come the roughly ridged shells of 



5. 



