3IO GEOLOGICAL BIOLOGY. 



growth in the circumferential direction. A general rule is,, 

 that the coarser plications are more prevalent among Silurian, 

 forms, while the forms with fine plications are more prevalent 

 in the Carboniferous. Increase in the actual number of pli- 

 cations on a shell is, as a variation of the species, due to ex- 

 tension of the hinge-line and corresponding parts of the shell, 

 and not to irregularity in the general size or number of the 

 plications upon a given extent of surface. 



H. Structure of Shell. — The shells of true Spirifers zxo. 

 fibrous in structure ; the presence of punctation characterizes 

 such closely allied genera as Cjrtina, SyringotJiyris, and Spiri- 

 fcrina. Cyrtina is present with the genus Spirifer in the 

 Niagara, and continues about as long as that genus. They 

 seem to be parallel genera, differing in the constant presence 

 of this character in the genus Cyrtina; but this peculiarity of 

 structure, the punctation of the shell, whatever it indicates, 

 is more conspicuous among the later than among the early 

 types of the family, and continues longer to be dominant. In 

 its first or initial appearance, as a character, it seems to have 

 been evolved intrinsically, among the distinctive differentia- 

 tions of the family. The modification of structure, which dis- 

 tinguishes punctate from fibrous structure, appears associated 

 w^ith other modifications and to involve considerable internal 

 adjustment. No evidence of the gradual appearance of the 

 character has been discovered. In Spirifcrina or Cyrtina the 

 punctation is found wherever, among the earlier forms, the 

 shells are well preserved. The punctate genera are sharply 

 distinguished from the types with fibrous shell structure. 



I. Surface Spines, Granulation, etc. — -These are associated, 

 more or less, with characters marked E, and affect the super- 

 ficial layer of the shell (the periostrachum) ; their development 

 is successive and accumulative, and is associated with particu- 

 lar series, and it appears to be a feature increasing with time, 

 both as to size and strength of the characters. The characters 

 develop quite in the extrinsic way in all the races in which 

 they have been traced. 



K. Special Development of the Median Septum. — This 

 modification in different species of the Spirifers is extrinsic in 

 its mode of evolution. One case has been traced with pre- 



