IX GEOLOGICAL CONTEMPORANEITY 293 



even the families of the Aporosa were all repre- 

 sented in the older Mesozoic rocks. 



Among the Mollusca similar facts were adduced. 

 Let it be borne in mind that Avicula, Mytilus, 

 Chiton, Natica, Patella, Trochus, Discina, Orbicula, 

 Lingula, Ithynchonella, and Nautilus, all of which 

 are existing genera, are given without a doubt as 

 Silurian in the last edition of " Siluria " ; while 

 the highest forms of the highest Cephalopods 

 are represented in the Lias by a genus Belemno- 

 teuthis, which presents the closest relation to the 

 existing Loligo. 



The two highest groups of the Annulosa, the 

 Insecta and the Arachnida, are represented in the 

 Coal, either by existing genera, or by forms 

 differing from existing genera in quite minor 

 peculiarities. 



Turning to the Vertebrata, the only palaeozoic 

 Elasmobranch Fish of which we have any complete 

 knowledge is the Devonian and Carboniferous 

 Pleuracantkus, which differs no more from existing 

 Sharks than these do from one another. 



Again, vast as is the number of undoubtedly 

 Ganoid fossil Fishes, and great as is their range 

 in time, a large mass of evidence has recently 

 been adduced to show that almost all those 

 respecting which we possess sufficient information, 

 are referable to the same sub-ordinal groups as 

 the existing Lcpidostcus, Polyptcrus, and Sturgeon ; 

 and that a singular relation obtains between the 



