340 GASTEROPODA OF THE INFERIOR OOLITE. 



the operculum of Neritopsis Bajocensis (PI. XXVII, fig. 10) shows no very obvious 

 signs of the process which should fit into the notch. Hence it must be remembered 

 that, so far as the Inferior Oolite of this country is concerned, one of the most 

 characteristic features in the diagnosis of the genus Neritopsis is wanting. The 

 most probable explanation is that the feature has been obscured through the 

 accidents of fossilisation ; but the absence of this character has induced some 

 conchologists of eminence, such as the late Robert Bell, to regard these forms as 

 belonging to Narica (Vanilcoro). 



Several species of Neritopsis are noticed by Moore as occurring in the Upper 

 Lias, and he also described Neritopsis cancellata, Moore, from the Lower Lias of 

 Brocastle. Seeing that specific differences, both in the Lias and Oolites, are 

 mainly based on variety of ornamentation, which evidently varied according to 

 locality as well as according to horizon, the precise biological value of these 

 distinctions may not be very great. In the Inferior Oolite of Dorsetshire specimens 

 run much larger than in other districts, but, on the whole, Neritopsis., though far 

 from being abundant, is by no means confined in its distribution. 



272. Neritopsis Bajocensis, d'Orbigny, 1850. Plate XXVII, figs. 9 a— c. 



Operculum, fig. 10. 



1850. Neritopsis Bajocensis, d'Orbigny. Prod., i, p. 264. 



1852. — — — Terr. Jur., vol. ii, p. 223, pi. ccc, 



figs. 8—10. 

 1867. — — — Laube, Gast. Br. Jura von Balin, 



p. 6, pi. i, fig. 9. 

 Won — — — Hudleston, G-eol. Mag., dec. 3, vol. i, 



p. 300, pi. ix, fig. 11. 



Description : 



Height of a medium specimen . . .23 mm. 



Width . . . . .26 mm. 



The following is the author's diagnosis : — " Shell oval, transverse. Spire very 

 short, composed of three very convex whorls, excavated near the suture, increasing 

 very rapidly, of which the last, of enormous size, is furnished spirally with 

 numerous small alternate unequal costse, and with radial undulations, like rounded 

 furrows." 



Relations and Distribution. — This species is especially characterised by the 

 thick undulating costa3 on the rounded shoulder of the body-whorl. It is 

 essentially a fossil of the Upper Division of the Inferior Oolite, and is almost 



