ONUSTUS. 329 
The proportions in this form are very similar to those of an average specimen 
of O. pyramidatus. The chief differences to note are that the cost are finer and 
more numerous, the scaly character remaining much about the same. Instead, 
however, of the nearly flat and unexcavated base, there is a very marked umbilical 
cavity. The base is plain, convex towards the centre, and presents towards the 
periphery a wide channel occasioned by the overlap of the margin of the body- 
whorl. It is true that this feature is partly shared by all varieties of the 
pyramidatus-group, but it seems especially developed in this form. 
Relations and Distribution.—In many respects this variety approaches O. 
lamellosus, d’Orb., more especially in the character of the base. None of our 
specimens exhibit the fine spiral striz round the umbilicus mentioned by the 
author, but the absence of these might easily be accounted for by the state of 
preservation. 
Very rare at Bradford Abbas; the figured specimen is from the Concavus-bed. 
262. Onusrus HELIACUS, d’Orbigny, var. opALINA. Plate XXVII, figs. 4a, 46. 
In this variety, which is rather narrower than the typical form, the spiral 
angle rarely attains to 70°. The figure is perfectly conical, rather reminding us 
of O. pyramidatus, from which species it obviously differs in its smaller habit of 
growth, in the wideness of the space between the axial coste, and in the more 
crenulated margin of the base. 
The base itself is smooth with fine radial lines. In the figured specimen very 
slight spiral striz are also noted. There is scarcely even a trace of an umbilical 
fissure, and this seems inconsistent with d’Orbigny’s description of O. heliacus. 
But Eugéne Deslongchamps tells us (‘ Bull. Soc. Linn. Norm..,’ vol. v, p. 132) that 
the so-called umbilicus in that species is a mere umbilical hollow. 
This variety of O. heliacus occurs on or about the Opalinus-zone at Drympton 
and Burton Bradstock. The majority of the specimens are in very bad condition, 
but the width between the costz in those cases where the crenulated margin is 
broken away will serve to distinguish it from all varieties of the pyramidatus-group. 
263. Onustus Heserti, Laube, 1867. Plate XXVII, figs. 6a, 66, and 7. 
1867. Onusrus Heserri, Zaube. Gastropoden des braunen Jura von Balin, p. 14 
(sep. copy), pl. iii, fig. 5. 
