LIFE. 497 



The progress was from the simple and straight form through the 

 coiled and complex, to the straight again, though without a return 

 to the original simplicity in these straight shells. The Nautilus, which 

 was associated with the Orthoceras in the Silurian, had the same sim- 

 plicity in its septa ; and this genus of chambered shells, out of the 

 forty or more that have existed, is the only one in our present seas. 

 The expansion of the type of Cephalopods was, therefore, not only 

 an increase in variety of forms and number of species, but also in 

 grade. With the Cretaceous period commenced the decline, and 

 at the close of the period there was a sudden falling off in families, 

 genera, and number of species. Whether any of the modern 

 Cuttle-fishes (Dibranchs) are equal, or superior, to the highest 

 Cephalopods of the Jurassic, it is difficult to determine. The mo- 

 dern genus Nautilus — representing the chambered species (Tetra- 

 branchs) — is certainly of far lower grade than the Jurassic Am- 

 monite. 



It is therefore one of the great facts connected with the Meso- 

 zoic era that in its later half the sub-kingdom of Mollusks passed 

 its period of culmination. But, while this is true of the sub-king- 

 dom as a whole, it is not true of each of its subdivisions ; for the 

 inferior tribes of Conchifers and Gasteropocls continue on the 

 rising grade through the Mesozoic, and probably have their maxi- 

 mum display in the age of Man. 



Culmination of the type of Ganoid Fishes. — The Ganoids, after the 

 Triassic, lose the Palaeozoic feature of vertebrated tails ; and this is 

 a mark of progress; for it is an example of that abbreviation of the 

 posterior extremity which generally marks elevation in grade as 

 well as progress in embryonic development. 



In the Jurassic period, the number of species of Ganoids reached 

 its maximum, and also the diversity of generic forms. The Ganoids 

 are at present nearly an extinct tribe. 



Over 300 Jurassic species have been described, besides nearly 50 Cretaceous, 

 and as many Triassic. The ordinary Rhombifers culminated in the Jurassic, 

 few of the genera continuing afterwards, while the Pycnodonts were numerous 

 in the Cretaceous, and continue to be largely represented in the Cenozoic. 



Culmination of the type of Hybodont Sharks. — The Hybodonts, which 

 began in the Palaeozoic, are numerous in Mesozoic species, espe- 

 cially in the Jurassic period, but almost extinct afterwards. Over 

 thirty Triassic and seventy-five Jurassic species have been de- 

 scribed. It is at present an extinct tribe. 



Culmination of the type of Labyriniliodonts, and therefore of that of Am- 

 phibians. — The scale-covered Amphibians, called Labyrinthodonts, 



