MORPHOLOGICAL DIFFERENTIATION. 337 



One genus, Aturia, is considered to be a distinct new genus 

 of the Tertiary; 16 genera were already well exhibited in 

 the Lower Silurian, or Ordovician. Only 8 genera lived 

 into the Devonian, only 5 to the Carboniferous, and but 2 

 (Orthoceras and Nautilus, the perfectly straight form and the 

 tightly coiled form) survived from Paleozoic into Mesozoic 

 time. 



The other suborder, Ammonoidea, has 94 genera; of 

 these, one genus is known as early as the Silurian (Goniatites), 

 one new genus (Clymenia) was added in the Devonian, and in 

 the latter part of the Carboniferous 5 more genera were initi- 

 ated. Of the rest, all appeared in the Mesozoic, 41 genera 

 beginning in the Triassic, 28 new genera starting in the Juras- 

 sic, and 1 8 new ones appearing, for the first time, in the 

 Cretaceous. Not a single genus of the whole suborder sur- 

 vived the Cretaceous period. Thus the Nautiloidea are 

 peculiarly Paleozoic in range, although there is still living the 

 genus Nautilus, and the Ammonoidea are peculiarly Meso- 

 zoic, and every genus of this suborder is now extinct. 



The other order, Dibranchiata, is less capable of showing 

 its history : the hard parts were of inferior character and less 

 in proportion to the fleshy parts, and upon the death of the 

 animal were much more likely to be destroyed 533 genera are 

 known, and all are Mesozoic, or more recent. There were 3 

 genera in the Jurassic, 15, Triassic, 8, Cretaceous, 10, Ter- 

 tiary and 3 now living. 



Second. The lesson, regarding the evolution of the ordinal 

 and subordinal characters and their generic expansion, which 

 we derive from these statistics is as follows : 



Rate of Differentiation of the Suborder Nautiloidea. The 

 Nautiloids (Orthoceras, Nautilus, and their kindred genera) 

 first appeared in the Ordovician. Before the close of the Silu- 

 rian this type had reached its fullest expansion, and began in 

 a very marked manner to drop out of the race ; 5 genera did 

 not survive from Ordovician into Silurian, and of the 22 Silu- 

 rian genera only 8 survived into the Devonian. Of this type 

 the two genera to live the longest were Orthoceras, the simp- 

 lest expression of the type, and Nautilus, probably the most 

 differentiated ; and the latter continued to live up to present 



