1 20 HELICID^E. 



strong ridges, one from the upper and one from the lower 

 extremity, gradually decreasing in height. Below these two 

 vertical plates there is a very thin horizontal fold terminating 

 posteriorly a little beyond the posterior vertical plate, and ante- 

 riorly becoming attenuated till it is scarcely visible at the parietal 

 ridge, to which, however, it is united. In the figure referred to, 

 I regret to find this horizontal fold is wrongly shown as termin- 

 ating a little beyond the anterior vertical plate. The palatal 

 armature consists of : first, a thin horizontal lamina, parallel with 

 and near to the suture, a little broader in the middle ; secondly, 

 a somewhat stouter lamina, slanting downwards a little posteriorly, 

 also a little broader in the middle, and abruptly decreasing 

 anteriorly, but very slowly posteriorly, where it is slightly in- 

 dented ; thirdly, a similar lamina, slanting a little more posteriorly, 



Fig. 58. Plectopylis ponsonbyi. 



with a slight indentation ; fourthly, a stout bilobed vertical 

 lamina, giving off anteriorly at the upper extremity a very slight 

 ridge and posteriorly from the base of each lobe a short ridge ; 

 fifthly, a horizontal fold parallel with and near to the lower 

 suture, raised in the middle, with the apical portion reflexed and 

 angular; it has a very small denticle on the posterior side. 

 Another very small denticle is situated a little below the first 

 horizontal lamina, about its middle, erroneously shown in a line 

 with it in fig. 58 d. Fig. 58 6 shows the whole of the armature 

 from the side of the aperture, fig. 58 c the same from behind and 

 fig. 58 d the inside of the outer shell-wall with the palatal folds 

 (all magnified) ; while fig. 58 e exhibits the shell restored, from 

 above, natural size. The type specimen is in Mr. Ponsonby's 

 collection and measures 18 mm. in diameter. A specimen sent 

 to me for examination by the late Dr. von Mollendorff, labelled 

 P. refuga, proved to pertain to the present species ; it measures 

 21, 17, and 7 mm. respectively, and the fold at the aperture is 

 longer and nearer to the ridge than is the case in the type. 



