ONUSTUS. 327 
Description.—The following are the proportions of the type specimen: 
Height : ; ; vo oo) 17 mam: 
Width ; : ; é . 23 mm. 
Spiral angle . 5 : : 2 80r: 
The spiral angle may be said to range from about 80° to 86°, and is slightly 
concave to nearly regular. Whorls six to seven in number; they are irregularly 
concave and more or less overlap, the line of junction being usually slightly 
crenulated. The coste are numerous, prominent, straight or slightly sinuous, the 
intercostal spaces being roughened by closely-set lamellx, which give a scaly 
appearance. This is much more conspicuous in some specimens than in 
others—a circumstance which may be attributed to differences in mineralisa- 
tion. 
The margin of the base is but slightly crenulated, the peripheral edge being 
almost plain, and so thin that the outline is seldom perfectly preserved in any of 
the specimens. The base has very fine radial lines, with occasional traces of spiral 
ones, but is otherwise smooth ; it is only slightly concave, rising again towards 
the columella. There is a semicircular umbilical excavation of moderate 
dimensions, but no umbilicus. Aperture very oblique. 
Varieties.—Of those to which it may be necessary to draw particular attention 
the most frequent is one which occurs in the Dogger and elsewhere, and which 
was mistaken by me for 0. ornatissimus, d’Orb, (vide ‘Geol. Mag.’ ut supra). 
In this form the costs are very short, and do not meet, so that the imbrication or 
overlap of the whorls is very conspicuous ; spire conical. This variety might be 
known as semicostata. 
Another variety from the Dogger is a very conical form, presenting the usual 
characteristics of O. pyramidatus, especially the flat and scarcely excavated base, 
but is absolutely devoid of costz. Whether or no this is a trick of mineralisation 
seems uncertain. This may be known as var. incostata. 
There are also in the Murchisonx-zone of Bradford Abbas and Halfway House 
some megalomorphs, which it would seem proper to regard as referable to this 
species, the more so as they exhibit the characteristic base; one of these must 
have measured 40 mm. in width. These seem to answer to Hugéne Deslongchamps’ 
description of Onustus lamellosus, @Orb. (vide * Bull. Soc. Linn. Norm..,’ vol. v, 
p. 133). 
Relations and Distribution.— Onustus pyranidatus may be regarded as the most 
abundant representative of the genus in the Jurassic rocks; distinguished from 
O. heliacus, d’Orb., by its larger habit, more closely-set coste, and more scaly 
surface, and also by the greater smoothness of the basal border. The same 
features will also serve to distinguish it from O. ornatissinws, d’Orb. 
It is not common anywhere, but occurs mostly in the Yorkshire Dogger 
