GASTEROPODA. 81 



infundibuliformi, umbilicum simulante; fissurd elongatd, antice clausd, non longius ab ore, 

 ultimum anfractum, subdepression perf or ante. (Deslongchamps.) 



Shell turbinated, conical ; whorls usually angulated, having a band or rib encircling 

 the middle of each whorl ; periphery subangular ; aperture basal, subquadrate; columella 

 curved ; base excavated, excavation large, and resembling an umbilicus ; fissure transversely 

 elongated, closed anteriorly, but not far from the outer lip, its length being about equal to 

 the distance which separates it. 



Our specimens exemplify the changes which the shell underwent during its advance of 

 growth. The perfect aperture, and likewise certain oblique furrows, to be seen upon other 

 parts of the shell, indicate so many stages of repose, each of which probably continued a 

 considerable period ; the amount of advance at each stage varied from one half to three 

 fourths of a volution. 



During the period of repose, the egress currents probably passed through the fissure ; 

 the edges of which are worthy of notice. The substance of the shell generally is thick, 

 but the edges of the fissure are extremely thin, and exhibit that irregular, ragged, or im- 

 perfect outline which is seen in bone or shell during the process of growth or absorption. 

 When, however, the animal was forming new shell in advance of the aperture, the fissure 

 was not advanced forward with it, but the anal siphon remained in the same position 

 until a considerable progress had been made in the formation of new shell. At length 

 that organ was withdrawn, to be protruded from the aperture, and the formation of a new 

 fissure immediately commenced. One specimen in our possession exhibits the fissure still 

 open, although the formation of new shell had proceeded beyond the old aperture to the 

 extent of one fourth of a volution. In this condition the outer lip is ragged and imperfect j 

 and during the brief period of the formation of a new fissure, the aperture acquires exactly 

 the aspect of a Pleurotomaria ; and it is not uncommon to find specimens in this condition. 

 The new shell is then very thin, and consequently is more or less crushed or imperfect. 

 These several removals of the anal siphon, and formation of new fissures at distant 

 intervals, are analogous to what is observed in Haliotis, except that in the latter genus 

 several perforations remain open during the formation of a new one, and their borders are 

 regular and smooth, not being destined to undergo the change which we observe in Tro- 

 chotoma. It seems indeed not improbable, that in the young state, or until three volutions 

 had been completed, that no fissure was formed, and that the siphon was protruded from 

 the aperture. This idea is founded upon the fact that those volutions are always smooth, 

 convex, and destitute of the encircling rib which subsequently follows the fissure. This 

 character is best seen by contrast in such species as in advanced growth become very 

 angular or step-like, as in T. tabulata, T. discoidea, and T. extensa. The reader is referred 

 to the interresting observations on this genus, by M. E. Deslongchamps, ' Mem. Soc. Lin. 

 Normandie,' vol.vii, pp. 99 — 104. 



