MORPHOLOGICAL DIFFERENTIATION. 33/ 



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

 of the Tertiary ; i6 genera were already well exhibited in 

 the Lower Silurian, or Ordovician. Only 8 genera lived 

 into the Devonian, onl}' 5 to the Carboniferous, and but 2 

 (Orthoceras and Nautilus, the perfectly straight form and the 

 tightly coiled form) survived ffom 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 18 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 ; 33 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 



