THE MESOZOIC AGES. 221 



the close of the period, when the ordinary horny- 

 scaled fishes, such as abound in our present seas, 

 appear to have been introduced. One curious point 

 of difference is that the unequally lobed tail of 

 the Palaeozoic fishes is dropped in the case of the 

 greater part of the ganoids, and replaced by the 

 squarely-cut tail prevalent in modern times. 



In the sub -kingdom of the Mollusca many im- 

 portant revolutions occurred. Among the lamp- 

 shells a little Leptaena, no bigger than a pea, is 

 the last and depauperated representative of a great 

 Palaeozoic family. Another, that of the Spirifers, 

 still shows a few species in the Lower Mesozoic. 

 Others, like Rhynchonella, and Terebratula, continue 

 through the period, and extend into the Modern. 

 Passing over the ordinary bivalves and sea-snails, 

 which in the main conform to those of our own 

 time, we find perhaps the most wonderful changes 

 among the relatives of the cuttle-fishes and Nautili. 

 As far back as the Silurian we find the giant 

 Orthoceratites contemporary with Nautili, very like 

 those of the present ocean. With the close of the 

 Palaeozoic, however, the Orthoceratites and their 

 allies disappear, while the Nautili continue, and are 

 reinforced by multitudes of new forms of spiral 

 chambered shells, some of them more wonderful 

 and beautiful than any of those which either pre- 

 ceded or followed them. Supreme among these is 

 the great group of the Ammonites, beautifully 

 spiral shells, thin and pearly like the Nautilus, and 



