NERITIDA. 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 cost, 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 lamella. 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 striz.” 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 O. 
pyramidatus and its allies by the coarseness and wideness apart of the axial costez 
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 Humphriesianus-zone at Oborne, which 
seems to connect rather with O. Heberti. 
In the width between the costa and in some other respects O. ornatissinus 
presents certain analogies with O. 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 Parkinsoni-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—NERITIDAR. 
Shell imperforate, globular, semiglobular, or patelliform ; spire very small, cavity 
simple owing to the absorption of the internal portions of the whorls ; aperture seii- 
lunate ; columellar side expanded and often flattened. 
In the Inferior Oolite of this country the following genera or sub-genera occur, 
viz. Nerita, Neridomus, and Pileolus. Of these Neridomws 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. 
