340 GEOLOGICAL BIOLOGY. 



Cyrtoceratidse. In America 2 genera (Cyrtoceras and Phrag- 

 moceras) began in the Ordovician and continue throughout 

 the Paleozoic, and Miller records of them 72 species in the 

 Ordovician era, and 42 Silurian, 20 Devonian, and 8 Carbo- 

 niferous species. 



Rate of Initiation of the Nautilidse. — Take the third type 

 (3), the discoidally spiral form Nautilus, and its various gen- 

 eric allies. The Nautilidss has in America 5 well-marked 

 genera. 4 genera, including 35 species, are Ordovician; 4 

 genera, including 17 species. Upper Silurian; 2 genera, in- 

 cluding 35 species, Devonian; 2 genera, including 62 species, 

 Carboniferous. In this case the apparently different law ex- 

 pressed in the number of genera and their decrease, and in the 

 number of species and their increase, is due to the combina- 

 tion in the family of two sets of genera, the one set of which 

 have their maximum representation of species early in the 

 Paleozoic ; the other increases in the number of its species as 

 we ascend. Litiiitcs, for instance, has 15 species in Ordovi- 

 cian, 7 in Silurian, and then became extinct. On the other 

 hand. Nautilus has 13 species recorded for the Ordovician, 4, 

 Silurian, 15, Devonian, 59, Carboniferous; and the genus con- 

 tinues on to the present time. 



History of Trochoceras by Species. — The helicoidal type (4), 

 including, for America, the one genus Trochoceras of the 

 family Trochoceratidae, is specifically represented as follows: 

 Ordovician i, Silurian 7, Devonian 10; and then it ceases. 



General Law of Evolution of Shell Cuvature in the Nautiloidea. 

 — Thus, to generalize, we find that this grand feature of the 

 Nautiloidea, the form assumed by the shell in its growth, 

 expresses the fulness of its differentiation among the repre- 

 sentatives of the first or initial period of the existence of the 

 race. All the several types of form run along together and 

 continue nearly, or quite, to the close of the Paleozoic, and 

 there, with the exception of two genera, become suddenly 

 extinct. 



Rate of Initiation of New Species in the American Region. — 

 Fourth. As if to emphasize the law above expressed regard- 

 ing the initiation of new genera, the statistics of the initiation 

 of species in the American rocks point in the same direction. 



