XV111 CONTENTS. 



Structure of shell Surface spines, granulation, etc. Special develop- 

 ment of the median septum, 301 Evolution of extrinsic specific char- 

 acters comparatively slow, although their plasticity is greatest at the 

 initial stage Laws of intrinsic and extrinsic evolution expressed in 

 variability and permanency of characters, 311. Hall's analysis of the 

 genus Spirifer and classification of its species Range of species of 

 Spirifer in American formations, 312. Each type of Spirifer shows a 

 continuous series of species, 313. Each of the chief types represented 

 at the initial period of the genus Three epochs of expansion, with 

 slow and gradual change during the rest of the history of the genus, 

 314. Characteristics of the life-history of Atrypa reticularis, 315. 

 Considerable and continuous plasticity of the species Nature and ex- 

 tent of the variations, 316. Hall's comment on the variability of the 

 species, 317. In the closing part of the life-period of the race the ex- 

 tremes of acceleration and retardation expressed Summary, 319. 

 Conclusions suggested by the study of Atrypa reticularis, 320. The 

 initiation of the species of Ptychopteria, 322. The law of progressive 

 evolution of Mammals, formulated by Osborne, 323. 



CHAPTER XVIII. 



THE RA TE OF MORPHOLOGICAL DIFFERENTIA TION IN A GENETIC SERIES, 

 ILLUSTRA TED BY A STUDY OF CEPHALOPODS. 



The evidence furnished by the Cephalopods Lankester's schematic Mol- 

 lusk, 325. Supposed characteristics of the primitive mollusk Differ- 

 entiation of the foot organ in mollusks, 327. The structure of the 

 Cephalopods, 329. Numerical rate of differentiation expressed in terms 

 of the initiation of new genera, 336. Rate of differentiation of the sub- 

 order Nautiloidea, 337. Mode of curvature of the Nautiloid shell 

 Rate of initiation of the Orthoceratidae of the Cyrtoceratidae, 339 of 

 the Nautilidae History of Trochoceras by species General law of 

 evolution of shell curvature in the Nautiloidea Rate of evolution of 

 new species in the American region, 340. Hyatt's formulation of the 

 law of rapid expansion of differentiation at the point of origin of a 

 new type of organism, 341. Summary, 342. 



CHAPTER XIX. 



PROGRESSIVE MODIFICATION OF AN EXTRINSIC CHARACTER; 



ILLUSTRA TED BY THE EVOLUTION OF THE 



SUTURE-LINES OF A M MONOIDS. 



The Ammonoids illustrate the law of acquirement of differences by grad- 

 ual modification, 344. Description of the characters of the Ammo- 

 noids, 345. Two divisions of the Retrosiphonatae : Goniatites and 

 Clymenias, 348. Quick evolution of the Clymeniidae Classification of 

 the Goniatites, 349. Differences in the sutures of the Ammonoidea 

 explained as various degrees of crimping of the edge of the diaphragms 

 Classification of the types of sutures (A) the Nautilian type of 

 suture (B) the Goniatite type of suture (C) the Ceratitic, Helictitic, 

 and the Medlicottian types of suture (D) the Ammonitic type of sut- 



