﻿43G 



show the apex to be blunt and rounded, but this rotundity may be 

 exaggerated in this part which had to be in part restored. 



The umbilical perforation is present, but it is very small. The 

 whorl grows very rapidly in all of its diameters and the bending of 

 the shell in the paranepionic substage is very abrupt, bringing the 

 continuation of this substage, the dorsum, in contact with the dor- 

 sum of the metanepionic and ananepionic parts of the first volu- 

 tion. 



In correlation with this, as in Trocholites, a distinct dorsal fur- 

 row appears as the shell bends in the first part of the paranepionic 

 substage. The coiling is so close that the slightest variation in the 

 same direction would obliterate the umbilical perforation. The 

 growing mantle while building the shell might have been influ- 

 enced by the proximity of the metanepionic dorsum and the small 

 diameter of the curve. The dorsal furrow here, as in Trocho- 

 lites, although occurring in the paranepionic substage before the 

 whorls touch, is perhaps due to the close contiguity of the whorls 

 and the rapid ingrowth of the primitive umbilical zones. This 

 process is still apparent in the first part of the second whorl, a sec- 

 tion of which is given immediately above the apex in Fig. 20. 

 This is the first of the ananeanic substage, and the siphuncle shifts 

 from its previously subventran position to propioventran. In the 

 metaneanic substage, in the latter half of the second volution, the 

 elongation of the ventro-dorsal diameters is faster, and the ten- 

 dency to develop lateral zones by the flattening of the sides becomes 

 marked. The sections of the whorls in the upper half of Fig. 20 

 are slightly distorted by compression, the lower half is in proper pro- 

 portion. The aspect of the section is better given in the more 

 enlarged Fig. 21, and the decrease in lateral diameters in proportion 

 to the ventro-dorsal is a marked characteristic and continues in the 

 ephebic stage. 



In some specimens this change is not so marked and the flatten- 

 ing of the sides develops later. 



In the later stages the siphuncle is slightly nearer the center as in 

 Fig. 19. 



Fig. 17 gives the full-grown ephebic stage, and is very close to 

 the original. The section Fig. 19 shows how closely this species 

 resembles Tarphyceras Champ lainense, differing only in the greater 

 rotundity of the venter and in the position of the siphuncle and in 



