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 costas 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-growp, but it seems especially developed in this form. 



Relations and Distribution. — In many respects this variety approaches 0. 

 lamellosus, d'Orb., more especially in the character of the base. None of our 

 specimens exhibit the fine spiral strias 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. Onustus heliacus, d'Orbigny, var. opalina. Plate XXVII, figs. 4 a, 4 6. 



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 0. pyramidatus, from which species it obviously differs in its smaller habit of 

 growth, in the wideness of the space between the axial costse, and in the more 

 cremilated margin of the base. 



The base itself is smooth with fine radial lines. In the figured specimen very 

 slight spiral strias are also noted. There is scarcely even a trace of an umbilical 

 fissure, and this seems inconsistent with d'Orbigny 's description of 0. heliacus. 

 But Eugene 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 0. 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 costse in those cases where the crenulated margin is 

 broken away will serve to distinguish it from all varieties of the pyramidatus-gvoxvp. 



263. Onustus Heberti, Laube, 1867. Plate XXVII, figs. 6a, 6b, and 7. 



1867. Onustus Heberti, Laube. Gastropoden des braunen Jura von Balin, p. 14 



(sep. copy), pi. iii, fig. 5. 



