﻿594 



first whorl, slight contact furrow of an ordinary transitional form, but 

 otherwise the nepionic stage resembles the adults of Aphetoceras in 

 its section and position of siphuncle and sutures. 



The gap between Pycnoceras and the next member of this series, 

 Tarphyceras, is wide and one or more genera are needed to fill up 

 the interval. 



In all of the genera mentioned above, except Tarphyceras, there 

 is no dorsal furrow, the zone of impression is produced by contact, 

 and the umbilical perforations are large. 



In Tarphyceras, however, although in form, sutures and position 

 of siphuncle the genus is closely allied to Aphetoceras, the young 

 are altogether distinct. 



As depicted on PL. iv, the young have very small umbilical perfo- 

 rations, the whorls broaden out by growth rapidly, and after a short, 

 straight or only slightly curved apical part is built in the ana- and 

 metanepionic substages, the broadening volution makes a sudden 

 and very abrupt gyroceran bend towards the apex. This is very 

 sudden and the umbilical perforation is flat or comma-shaped. 



It might of course be shown, if other intermediate shells were 

 found, that the mechanical effects of this sudden bending did not 

 produce the dorsal furrow, but that this is an adequate mechanical 

 cause can reasonably be claimed by those who oppose the view that 

 it is due to heredity. 



It has already been shown that the outer side or venter tends to 

 grow faster than the inner, and if this reaches a point in its ratio of 

 growth that far exceeds that of the inner side, it is obvious that it must 

 act upon that side as a force that bends or tends to make it more 

 arcuate in proportion to this excess of growth or rapidity of increase. 

 The outer side being free would be apt to retain its genetic 

 form, and the inner side or dorsum would be greatly influenced or 

 moulded by the pressure to which it was subjected. Thus it can be 

 assumed that in case of a sudden bending, as in Tarphyceras, the 

 venter would maintain its rounded outline and forcing the dorsum 

 inward as it grew would tend to make it assume the arcuate form or 

 bend inwards in a crease or dorsal furrow in the paranepionic volu- 

 tion conforming more or less with the shape of the dorsum of the 

 metanepionic volution. 



There are some reasons why this explanation is not wholly satis- 

 factory. In the first place, if this be the case, why did not the 

 whorl of the paranepionic completely close the umbilical perfora- 



