148 HELICID^E. 



arm, provided anteriorly at the lower extremity with a short, 

 abruptly descending horizontal ridge ; the fold then rises obliquely 

 for a short distance, and finally bifurcates ; the lower arm of the 

 bifurcation the shorter, and descending almost vertically ; it is 

 provided posteriorly with a short horizontal ridge at its lower 

 extremity ; the upper arm at first ascends obliquely, then proceeds 

 horizontally close to the suture, and gradually attenuates. Below 

 these complicated structures, there is a free, thin, horizontal fold 

 close to and parallel with the lower suture, and extending from, 

 the aperture to a little beyond the lower arm of the bifurcation 

 and its posterior support (see fig. 78 e, which shows part of the 

 parietal wall). At the aperture this fold is distinctly united to the 

 transverse sinuous ridge (see fig. 78 d). The palatal armature con- 

 sists of : first, a strong long horizontal fold near the suture and 

 parallel with it, as well as with the posterior portion of the upper 

 arm of the parietal bifurcation, with which it terminates at the 

 same point posteriorly ; secondly, a shorter, but much stronger 

 and broader horizontal fold, which deflects with a sharp curve 

 posteriorly, having a little above its posterior termination, and 

 almost in a line with its anterior portion, a slight elongated 

 horizontal denticle ; thirdly, a very short, but strong and broad 

 crescent-shaped fold, deflected at both extremities ; fourthly, facing 

 the concave side of the last-mentioned fold, is a very strong and 

 broad vertical lamina, strongly inclined towards the aperture, with 

 a much reflected and thickened edge,- this lamina intercalates 

 between the two lower arms of the parietal armature ; on the 

 posterior side of the lamina and near its lower extremity occurs a 

 stout little denticle, and a little lower and still farther back is 

 found a slight elongated swelling, not amounting to a fold or 

 denticle (yet present in all four mature specimens, as well as in 

 an immature one, examined by me) ; fifthly, a thin horizontal 

 fold, the anterior part straight, but curved in the posterior half, 

 with the concave side facing the vertical lamina (see fig. 78/, 

 enlarged, which shows the inner side of the palatal wall with its 

 folds and denticles). Figs. 7&g-i (also enlarged) show an 

 immature specimen of 5|- whorls, in Mr. Ponsonby's collection ; 

 the armature is almost identical with that of the mature specimens, 

 but the main median parietal fold is very short and does not rise 

 from the aperture, while the denticle in front of the lower part of 

 the palatal vertical lamina is very strongly developed, and it is 

 united to it so as to form a steep ridge. A second set of barriers, 

 identical in every respect except in being a little smaller, occurs 

 in this specimen | of a whorl further back. The mature specimen 

 shown in fig. 78 d is also in the collection of Mr. Ponsonby, 

 and measures : major diam. 31, minor 24, alt. 9 mm. ; while 

 the immature specimen measures 17 mm. in diameter. Three 

 specimens in my collection measure respectively 29 : 23 : 9 mm. ; 

 25 : 20 : 8'5 mm*. ; 23 : 18 : 7*5 mm. The types of the species are 

 in the New York State Museum, at Albany, N.T., and are shown 



