NERITIDiE. 331 



The following description by d'Orbigny suits the British specimens very well. 

 " Shell much wider than high, considerably umbilicated. Spire formed by a 

 concave angle, excavated, composed of whorls somewhat concave, furnished axially 

 with straight costae, radiating from the summit of the spire towards the margin, 

 where they are terminated by a point. In the interspaces are fine oblique trans- 

 verse lamellae. The last whorl, somewhat convex above, carinated, and with points 

 on the periphery, is umbilicated in the centre, and around this occurs a prominence 

 with concentric striae." The aperture is rhomboidal and depressed, as in all 

 species of Onustus from the Jurassic rocks. 



Relations and Distribution. — This species is easily distinguished from 0. 

 pyramidatus and its allies by the coarseness and wideness apart of the axial costae 

 and the serrated character of the basal periphery, which in some specimens 

 almost equals the figures in the ' Terrains Jurassiques.' Fig. 8 of the accom- 

 panying plate represents the usual British form, whilst fig. 7 represents a peculiar 

 form occurring in the upper part of the Humphriesiamts-zone at Oborne, which 

 seems to connect rather with 0. Heberti. 



In the width between the costae and in some other respects 0. ornatissimus 

 presents certain analogies with 0. heliacus and its varieties, which it also more 

 resembles in size ; but it is distinguished from that species by the presence of a 

 distinct umbilicus, the edge of which is spirally striated, and also by the much 

 stronger serrations of the basal periphery. Typical specimens are somewhat 

 abundant in the ParJcinsoni-zone of Burton Bradstock and Vitney Cross, but very 

 rare elsewhere, and almost unknown much further north. 



Var. Pontonis, Morris. — Specimens from the Lincolnshire Limestone are in such 

 a poor state of preservation that it is difficult to say what their relations may be. 

 I have a specimen from Ponton which seems to be a small variety of this species, 

 but as the base is not seen the identification cannot be regarded as certain. 



Family— NERITULE. 



Shell imperforate, globular, semiglobular, or patelliform ; spire very small, cavity 

 simple owing to the absorption of the internal portions of the whorls ; aperture semi- 

 lunate ; columellar side expanded and of ten flattened. 



In the Inferior Oolite of this country the following genera or sub-genera occur, 

 viz. Nerita, Neridomus, and Pileolus. Of these Neridomus is perhaps best considered 

 as a section of Nerita allied to Neritina. Shells of this family are far from 

 abundant in our Inferior Oolite. They mostly occur on a low horizon, and appear 

 to be wanting in the rich shell-beds of the Dorset-Somerset district. 



