Essay introductory to Geology. 9 



so speak) the placoidians and gonoidians in the beds beneath the chalk ; 

 and of these fossil remains, not only the species, but all the genera likewise 

 are now extinct, the orders alone remaining. Many species of the shark- 

 like placoidian, with ichthyodorulites, are found in the various formations 

 of this groupe 5 but the species contained in different formations, are in 

 every case distinct ; this is indeed a general law with regard to the dis- 

 tribution of fossil fishes. The gonoidians are characterised by possessing 

 a symmetrical caudal fin. 



The chalk, with regard to its fish, approximates more to the character of 

 the superincumbent tertiary groupe, it presenting for the first time genera 

 of the more recent orders of ctenoidians and cycloidians j but still the 

 genera occurring are for the most part, though not universally, extinct. 



The saurian reptiles now make their first decided appearance, and seem 

 to have been most abundant ; and in the variety of their genera and species, 

 to have greatly outnumbered the ])resent state of this class. In the con- 

 tinental equivalent of our magnesian limestone, monitors appear ; and at 

 Bristol remains have been recently found, intermediate between the cro- 

 codile and megalosaurus. In the lias we have those vast marine monsters, 

 the ichthyosauri and the long necked plesiosauri, mingled with a species 

 of gavial, widely distinguished from the recent. Either species of all these 

 genera occur in the superincumbent oolites, together with the mososaurus 

 and the megalosaurus ; and in the sandy limestone of the weald, a sin- 

 gular saurian, armed with a horn, and possessing the teeth of the iguana, 

 is likewise found. And to these saurian reptiles, we may add the remains 

 of a species of turtle. 



Of terrestrial mammalia, one single specimen, the jaw of an opossum, 

 has been found in the slaty oolite of Stonesfield, near Oxford ; but this 

 sufficiently attests the existence of these higher animals during this geo- 

 logical period, and clearly proves that we must not infer that such species 

 had not been created, because we do not find these remains where in truth 

 we can hardly expect to find them, in these deposits of an ancient ocean. 

 It may perhaps hereafter be found that certain portions of these deposits 

 have, more than others, the characters of literal deposits ; and in these 

 we should be most likely to recognize the remains of terrestrial animals. 



Although the low organization of insects would have required us, in a 

 strict method, to speak of them much earlier, yet as terrestrial animals we 

 may here notice, that in the same Stonesfield quarries last mentioned, the 

 elytra of coleopterous insects are found ; and that very perfect species of 

 libellulae occur in the lithographic calcareous slate of Solenhofen. 



As to vegetation, this order is said to require a subdivision into two 

 stages ; the lower including the paecilitic groupe, and the upper the oolitic, 

 and cretaceous. The poecilitic vegetation includes about twenty species, 

 differing alike from those of more ancient and more modern formations. 

 The cryptogamia: arc less numerous, and of smaller size than in the more 



No. I.— Vol. I. c 



