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rapidly until it reaches the centrodorsan position in the anephebic 

 substage at the beginning of the third whorl. 



The ventro-dorsal diameters also slowly decrease by growth cor- 

 rectively with this movement, along the mesal plane and proceed 

 with equal steps, correlative with changes in the septa, and relative 

 dimensions and shapes of the air chambers and the shifting of the 

 siphuncle towards the dorsum to the first quarter of the third whorl 

 where they take on the adult proportions and aspect. 



These facts are admirably well shown in the figures of Schroedero- 

 ceras {Lit.) teres by Holm, reproduced here if allowance is made 

 for the more cyrtoceran or less involute form of Eatoni, which 

 has a larger umbilical perforation. The third septum in both 

 forms, however, comes internally to the same point, the end of the 

 cyrtoceran stage, when the whorl makes a sudden bend and assumes 

 the gyroceran curvature that brings it at the end of the first whorl 

 against apex of the conch. In Figs. 21 and 22, from Holm, this 

 bend is more abrupt and more like that of Trocholites than in this 

 species. The dorsal side of the last quarter of the first whorl 

 actually strikes and lies upon the dorsal side of the first air cham- 

 ber, whereas in this species the contact takes place farther towards 

 the apex. In teres also, according to Holm's figures, the approxi- 

 mation of the siphuncle towards the dorsum takes place more 

 rapidly and probably earlier than in Eatoni. Holm found no signs 

 of a cicatrix on the apex of teres t but no shell is represented in his 

 figures and he describes the whorls as so very closely approximated 

 that there was but one shell wall. The young shell is very thin, 

 and probably this explains the difficulty of separating the whorls. 

 At any rate, the absence of the cicatrix is not established by his 

 observations. I think he must have overlooked the shell wall, this 

 not being absent in any other forms that I have examined. 



Fig. 34, PL vi, gives the aspect of an accidental section, the 

 location of which is shown by the line through Fig. 35, taken from 

 the center of Whitfield's original of this species. The sections 

 passed subdorsan to the shell, cutting across the two first septa of 

 the metanepionic substage. The peculiar aspect of this part of the 

 section is due to the continuity of the lateral shell lines on either 

 side with those of the paranepionic whorl which is given in section 

 of volution immediately under this. The convex line dividing the 

 metanepionic from the paranepionic volution, the projecting third 

 septum. The reverse, the splinter from which this section was 



