DEVELOPMENT OF THE SHELL IN TORNOCERAS 439 



annular lobe, although the two latter are less strongly marked 

 than the others. Further growth merely serves to emphasize 

 these features, until the neanic stadium, when the ventral 

 lobe is extended by the siphonal fissure, and a minute cone 

 appears at the bottom of the annular lobe. 



Several specimens of the protoconch give evidence of the 

 presence of the siphonal caecum, and show that it was 

 probably closely appressed to the ventral wall. Figure 3 

 illustrates the ovoid marking on the interior of the shell, 

 enclosing two diverging lines which apparently represent the 

 appressed portion of the true caecum, while the outer curved 

 lines limit the shelly deposit of attachment. The relative 

 diameter of the siphon at the first and for a number of suc- 

 ceeding chambers is much greater than in the mature shell 

 (figures 1, 6, and 13). From the beginning, it is situated 

 close to the abdominal wall, and is nearly invariable in its 

 character. 



The embryonic shell is very thin, and almost smooth in its 

 earlier portions ; then fine revolving lines of granules appear, 

 which become progressively more pronounced and arranged 

 in transverse rows, between which the earliest of the concen- 

 tric striae are developed. With the increase in the strength 

 of the striae, the granules disappear, and are obsolescent 

 before the protoconch is completed (figure 7). The striae are 

 sharp, elevated, and straight, forming a conspicuous feature 

 of the ornamentation, until in the third or fourth whorl, 

 when they become subdued, and finally are replaced by the 

 fine inconspicuous and often fasciculate lines of growth which 

 are present in the adult shell. No indication of a funnel is 

 shown up to the completion of the first whorl (figure 10), as 

 the striae continue straight across the ventrum, but in the 

 second (figure 11) the pronounced sinus in the striae shows 

 that the funnel had developed or, at least, had become of 

 functional importance. 



A comparison of the figures of T. retrorsum von Buch, var. 

 typum Sandberger, as illustrated by Branco (loc. cit., pi. v, 

 fig. vii), with the present species shows that the former 



