190 LAY SERMONS, ESSAYS, AND REVIEWS. [x. 



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 Pleuracanthus, 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 Lepi- 

 dosteus, Polypterus, a nd Sturgeon ; and that a singular relation 

 obtains between the older and the younger Fishes ; the former, 

 the Devonian Ganoids, being almost all members of the same 

 sub-order as Polypterus, while the Mesozoic Ganoids are almost 

 all similarly allied to Lepidosteus. 1 



Again, what can be more remarkable than the singular con 

 stancy of structure preserved throughout a vast period of time 

 by the family of the Pycnodonts and by that of the true Ccela- 

 canths : the former persisting, with but insignificant modifica 

 tions, from the Carboniferous to the Tertiary rocks, inclusive ; 

 the latter existing, with still less change, from the Carboniferous 

 rocks to the Chalk, inclusive ? 



Among Reptiles, the highest living group, that of the 

 Crocodilia, is represented, at the early part of the Mesozoic 

 epoch, by species identical in the essential characters of their 

 organization with those now living, and differing from the latter 

 only in such matters as the form of the articular facets of the 

 vertebral centra, in the extent to which the nasal passages are 

 separated from the cavity of the mouth by bone, and in the 

 proportions of the limbs. 



1 &quot; Memoirs of the Geological Survey of the United Kingdom. Decade 

 x. Preliminary Essay upon the Systematic Arrangement of the Fishes of 

 the Devonian Epoch.&quot; 



