GASTEROPODA. 57 



examples being such as usually have the most widely-separated longitudinal lines or striae. 

 The surface markings vary so considerably that they may be conveniently described under 

 the three following aspects : — 



d . Ribs elevated and separated ; carinas smooth and prominent. 



b'. Surface with longitudinal closely-arranged plications, but no distinct ribs; plicae 

 impressed with very fine longitudinal lines. 



c. In which the characteristics of the two former varieties are sometimes exhibited 

 upon the same specimen, in which case the carinas are imperfect, or, in lieu of them, there 

 are slight depressions or furrows. 



The most common aspect is that of the variety a. It is one of the most generally- 

 noticed univalves of the shelly beds. Very rarely all traces of ribs and plications are 

 wanting, the surface is then shining, smooth, and highly coloured. The dimensions vary 

 from that of a duck-shot to the largest-sized pea. 



Locality. Minchinhampton Common. In all the shelly beds of the district. 



Nerita costulata, Desk. Plate VIII, figs. 6, 6a,b,c. Plate XI, figs. 18, 18a, b. 



Nerita costata, Sow. 1824. Min. Con., t. 463, figs. 5, 6. 



— — Phillips. Geol. of Yorkshire, vol. i, t. 11, fig. 32. 



— — Brown. Illust. Foss. Conch., p. 91, t. 44, figs. 1, 2. 



— costulata, Desk. 1838. Lam. Anim. sans Vert., 2d Edit., vol. viii, p. 617. 



— — D'Orb. 1850. Prod. Paleont., p. 299. 



N. Testa parvd; spird obligud depressd, minutd, vix elatd; anfractu ultimo superne 

 planato, costis longitudinalibus, numerosis, subundatis, et approximatis ornato. 



Shell small; spire oblique, depressed, minute, scarcely elevated; the last whorl 

 flattened upon its upper portion, and covered with costae, which are longitudinal, 

 numerous, closely arranged, and slightly waved ; the aperture is very large, the inner lip 

 very wide and flat. 



The absence of carinae at once distinguishes this little shell from our Nerita rugosa, for 

 one variety of which it might otherwise be mistaken. It has not been found in the 

 Minchinhampton Great Oolite, but occurs occasionally in the Inferior Oolite of that district. 

 Axis 2 lines. 



Locality. Ancliff; Stonesfield. Inf. Ool., Yorkshire. 



Sect. 2. Inner lip convex. Neridomus. 



N. Testa laevigata, ovato-globosd ,- spird parvd, obligud; anfractu ultimo permagno; 

 aperturd ovatd, vel semilunari; labio externo crasso; labio interno crasso, convexo et 

 Icevigato. 



Shell smooth, ovately globose; spire small, oblique; the last whorl very large; aperture 

 ovate, or semilunar ; outer lip thick ; inner lip thick, convex, and smooth. 



