4 i4 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



impressed zone is not present in the young of Ophidi- 

 oceras, the closest coiled of all these forms, nor in the 

 young of most species of the Silurian before the whorls 

 touch, and all of the species likely to present this pecu- 

 liarity have been investigated. 



"The impressed zone is also, as a rule, lost in the 

 oldest stage of the whorl of every individual when the 

 whorls cease to continue to grow in contact. This 

 condition is represented in the last part of the outer- 

 most whorl of Nos. 4 and 5 in sections, Nos. 4^, 5 e, 

 and in the outlines of their apertures, which are ellip- 

 tical. The sections represent cuts through the whorls 

 when, as is the case in extreme age, these cease to in- 

 crease in size. ' As soon as this senile contraction be- 

 gins to occur, the sides shrink, becoming narrower, 

 the amount of involution becomes less, and the im- 

 pressed zone shrinks in breadth, as shown in the sec- 

 tions. When the whorl finally parts company in 

 consequence of continued contraction, the already 

 shrunken impressed zone, Nos. ^d, $d, rapidly dis- 

 appears, and the apertures of such shells are frequently 

 as round and free from indentations on the inner as on 

 the outer side, as is shown at the free end of Nos. 4 

 and 5. 



"In normally uncoiled forms, usually named Litu- 

 ites, when the adult or young is coiled, and the suc- 

 ceeding stages, whether representing adults or old 

 shells, are uncoiled, the phenomena are similar. The 

 impressed zone is lost after the growth ceases to bring 

 the whorls of the shell into contact. 



" The young and the adults of many of these forms 

 have now been observed in the earliest periods, and it 

 is, therefore, obvious that during these early times the 

 impressed zone must have been a modification of the 



