216 GASTEROPODA OV THE INFERIOR OOLITE. 



152. Nerin^a (Ptygmatis) xbnos, sp. nov. Plate XV, figs. 4 a, 4 h. 



Description : 



Spiral angle (regular) . . . . 4°. 



Height of wliorl to width (approximate) . .1:1. 



Shell cylindrical, slightly turrited. Whorls tolerably numerous, for the most 

 part flat, but slightly constricted about three -fourths of the way down. The 

 suture is situated in the middle of a sutural belt of moderate prominence, 

 which becomes almost effete in the later whorls. Sutural angle oblique. The 

 ornaments consist of numerous fine spiral lines, of which one, rather larger than 

 the rest, occupies the hollow of the constriction ; they seem to fade away in the 

 later whorls. 



The section shows five principal folds of a peculiar character. The anterior fold 

 on the outer wall is much extended longitudinally ; it is broad-headed, and deve- 

 lops slight subsidiary folds ; the posterior fold is small and curved. Of the folds 

 on the columellar side, the lowest is much extended longitudinally, the second is 

 slight, the third (on the posterior wall) is narrow and deep. 



Relations and Distribution. — The whorls are relatively higher than in Ptyg. 

 pisolitica, and the peculiarly extended character of the lower folds of the section 

 still further helps to differentiate it from that species, to which it is probably the 

 most nearly allied. There seems also to be some slight differences of ornamen- 

 tation, but the indications are obscure, even if they be of much specific value. 



Ptyg. xenos is interesting as occurring on the lowest horizon of any Nerinaea 

 yet discovered in the Cottes wolds. It is met with somewhat sparingly in the 

 shell-bed below the Lower Limestone on Crickley Hill, which is on the border-land 

 between the Miircliisonse- and the Opalinus-zone. Whether this species or Nerlnsea 

 cingenda is the oldest member of the genus in England remains yet to be deter- 

 mined ; if the Nerinssa-hed of the Dogger is strictly on the Pea-grit horizon, as 

 supposed by some, we must regard Ptgy. xenos as the oldest Nerinxa at present 

 known in this country. 



The following group of species has this much in common, viz. that each 

 possesses, or is presumed to possess, an internal structure very similar to that 

 of Nerinsea Oppelensis, Lycett. The section has seven folds, but although the 

 folds are so numerous, they do not seem to restrict the space so much as is 

 the case in some other species of Ptygmatis presently to be described. This 

 group appears to be represented in the Bathonian of France by Nerinxa, 

 bacillus, D'Orb. 



