W. H. Hudleston — Gasteropoda of the Portland Rochs. 393 



8. Chemnitzia decussata, sp.n. Plate XT. Figs. 11a, 116. 



Description. — Specimen from the C?/rena-beds of Chilmark Quarry. 

 This form differs from the one last described chiefly in the more 

 strongly defined belt encircling the upper portion of each whorl, and 

 in the ornamentation, which is very obvious at the base of the shell ; 

 the aperture also is more oval, and there are some traces of an 

 umbilicus. 



Belations and Distribution. — The belt at the posterior extremity of 

 each whorl rather serves to remind one of Chemnitzia teres, but 

 Ch. decussata is more stumpy in its growth, besides showing consider- 

 able traces of ornamentation. The form figured, 12a and 126, may 

 perhaps be referred here. Found sparingly in the Cyrena-heds of 

 the Vale of Wardour. 



9. Neritoma sinuosa, Sow., 1818. Plate XI. Figs. 5a, 5b. 



JVerifa sinuosa, Sow. 1818, M.C., pi. 217. 



Mrita angulata. Sow. 1835, Trans. Geol. Soc. 2ndser. vol. iv. p. 347, pi. 23, fig. 2. 

 JS'eritoma sinuosa, Morris, 1849, Q.J.G.S., vol. v. part 1, p. 334, with figiu-e. 

 Neritoma sinuosa, Morris (Sow.), 1864, De Loriol et Pellat, Port, de Boulog., p. 35, pi. 



iii. figs. 19-21. 



No description of this well-known species is needed after the very 

 full account given by Morris {op. cit.) in his article on Neritoma. 

 The species is also well described and figured by De Loriol. As the 

 companion, in so many cases, of the fossils to which this communica- 

 tion relates, a glance at its relations and distribution may not be out 

 of place. 



Belations and Distribution. — Neritoma would seem to be eminently 

 representative of that sort of generic confusion which prevailed at 

 the time when the Portland Kocks of England received their fossil 

 contents. De Loriol {op. cit. p. 37) is of opinion that, in some 

 respects, the shell recalls the peculiarities of certain Naticas, and he 

 even considers that the animal may have approached that type. As 

 a subgenus it is very limited both in range and number of species. 



The distribution of Neritoma sinuosa is peculiar. It is extremely 

 abundant in the Cyrena-heds of the Vale of Wardour, and is also 

 found more sparingly in the upper Cyrena-heds (No. 1) of that 

 locality. In the highest Portlandian bed at Swindon, which belongs 

 to the Cyrena type, very large specimens occur. In Bucks it is 

 rare, and has only been found in the very topmost beds at Quainton. 

 Of its presence in the Eoche of Portland I am not quite certain. It 

 is hardly necessary to mention that this species does not occur in 

 the more purely marine Portlandian beds of any locality. 



When we trace its distribution on the Continent, we find that, at 

 Boulogne, Neritoma sinuosa is stated to be pretty common in the 

 Lower Portlandian (zone of Am. gigas), but is not quoted from the 

 Upper Portlandian — usually held to be the equivalent of our 

 Portland Stone. In Hanover, according to Struckmann, Neritoma 

 sinuosa (= Nerita ovata, Koem.) is most abundant in part of the 

 Middle Kimmeridge. Even granting that Neritoma ovnta is sufficiently 

 different to be regarded as a distinct species — the view taken by 

 De Loriol — we cannot fail to notice with interest the fact that 



