TF. R. Sudleston — On the Yorkshire Oolites. 51 



tricks which mineralization sometimes plays with the fossils of the 

 Dogger.) The whorls are smooth — almost polished in the specimen 

 — but a fine S3'stem of undulating spirals may be seen on close 

 inspection, and these are decussated by broad flat lines of growth. 



The aperture is wide, and subcircular, the outer lip being produced 

 and slightly pouting ; columella nearly straight and considerably 

 thickened. 



Relations and Distribution. — The above forms are by no means 

 easy to allocate generically, and it is quite open to question whether 

 the varieties A. and B. should not be more widely separated. 

 Variety A., which is the least rare of the two, has the shell extremely 

 depressed and neritoid, and the ornamentation so characteristic of 

 the more delicately sculptured amongst the Jurassic Neritopsids is 

 conspicuous in a good light without the aid of a lens. In this 

 respect variety A. approaches Neritopsis [Tarho) canaliculatus, 

 D'Arch., from the Cornbrash of Yorkshire, described in the last 

 number of this Memoir.^ There are a fair number of specimens 

 in the best collections, but hardly any afford opportunities for a 

 study of the aperture. The Jermyn-Street specimen (Fig. 2) is 

 almost alone in this respect, and even in this one the conditions 

 are not altogether favourable for a very accurate diagnosis. The late 

 Dr. Lycett appears to have regarded this specimen as a Neritopsis. 



Variety B. is very like Phillips's figure, and also like D'Orbigny's. 

 Although it has many points of resemblance with A., yet there are 

 important differences. The spiral angle is 15^ less and the ratio 

 of the body- whorl 10 per cent, less, and the aperture is relatively 

 smaller, and more circular. Indeed the whole aspect of the shell is 

 less like a Neritopsis. This variety must be regarded as the re- 

 presentative of Phillips's type. It is, however, very rare. 



Both varieties occur sparingly in the Dogger, but as yet I have not 

 identified them from the Inferior Oolite of the West of England. 

 Supposing that variety B. is really represented by Delpliinula gibbosa, 

 Thorent, it occurs in the Inferior Oolite of St. Michel (Aisne), 

 France. 



Genus Turbo, Linnaeus, 1758. 



As previously intimated in the " Corallian Gasteropoda " (Geol. 

 Mag. 1881, p. 52), and also in a former part of the present Memoir, 

 owing to the withdrawal of Amberleya, and the so-called Littorince., 

 the number of species left to be classed under Turbo is limited. If 

 we were further to withdraw the forms usually referred to Monodonta 

 and Delphinida, there would be no representative of Turbo at all in 

 the series of beds now under consideration. It must be admitted 

 that, in the absence of certain characters easily recognized in existing 

 species, it is extremely difficult to know how to treat the quondam 

 Turbos of our lists : nay more, it is not always easy to say (when the 

 aperture is concealed or imperfect) whether a species should be classed 

 under Turbo or Trochus. Hence I am free to admit that the arrange- 

 ments at present adopted are somewhat empirical, and thus the list of 



Geol. Mag. Dec. III. Vol. I. p. 301. 



