PLBCTOPYLIS. 85 



perceptible below, where the shell is also more shining than 

 in P. plectostoma. It is translucent and the armature is 

 distinctly visible through its wall, while the aperture is more 

 narrowed laterally and the upper margin of the peristome is less 

 arcuate, being a little inflected. The umbilicus is also wider 

 and scarcely angulated, while the base is much more flattened. 

 The ridge of the parietal callus is more raised and more curved. 

 The parietal armature consists of a vertical plate with a very short 

 support anteriorly at the upper and lower extremities, but without 

 the horizontal fold above as in P. plectostoma. The two denticles 



\:i 



d 



Fig. 34. — Plectopylis affinis. 



on the posterior side are larger and more elongated, and below 

 the vei'tical plate are two short, thin, horizontal folds in a line 

 with each other (see tig. 34 d, wiiich shows the parietal wall by 

 itself ; and fig. 34 c, which shows both armatures from the posterior 

 side). The palatal armature is similar to that of P. plectostoma, 

 but the posterior portions of the third, fourth, and fifth folds, 

 instead of being straight and almost vertical, are crescent-shaped 

 and oblique (see tig. 34 6, which shows the palatal folds as they 

 appear through the shell-wall); an additional semi-circular fold, 

 posterior to, but a little above, the fifth fold, occurs in this 

 shell ; this, however, 1 have not observed in any of the other 

 specimens. Fig. 34 « shows the entire shell enlarged. My 

 specimens were obtained from Mr. Fulton some years ago ; the 

 twenty-five further specimens from the same locality, sent to me 

 for inspection by him, range from 9 to 11 milhmetres in diameter. 

 Two immature specimens in my collection are composed of five 

 and a half whorls; one of these has the immature barriers 

 complete, but the palatal folds are very short and the posterior 

 oblique portions of the fourth and fifth folds are almost straight 

 instead of crescent-shaped ; externally a slight trace of previous 

 folds can be discerned ; in the other specimen the last immature 

 folds are similar to those of the fii'st specimen, but the remains of 

 a previous set are in a less advanced stage of disintegration. 



