ONUSTUS. 325 
Genus—Onvstus, Humphrey, 1797. 
We owe to the late Hugéne Deslongchamps, in the first instance, the recognition 
of the fact that several trochiform shells found in the Jurassic rocks, which had 
been referred by d’Orbigny to the genus Trochus, might with more propriety be 
classed under the genus Onustus. Deslongchamps published his conclusions on 
this subject in 1860,' and pointed out that in the Jurassic rocks of France the 
genus had been noted in the Middle Lias, the Upper Lias, the Inferior Golite, 
the Great Oolite, and the Callovian. Lycett in 1863° accepted these conclusions, 
and gave a diagnosis, based upon that of Deslongchamps, more especially 
applicable to the Jurassic species. 
Shell conical, wide-angled, whorls flattened or rendered somewhat concave by an 
expansion of their lower borders, which overhang and conceal the suture ; the lower 
border of the last whorl is produced horizontally ; surface irregularly striated. 
The base is concave towards the outer border and convex towards the centre; the 
umbilical orifice is sometimes large but in other instances small, and becomes nearly 
concealed by advance of growth ; the aperture is depressed and oblique, columellar 
side curved, outer lip angular and produced.—Modified after Lycrrr. 
As observed by Lycett, none of the Jurassic species exhibit those agglutinations 
of shells and stones which are so characteristic of the Tertiary and Recent 
examples of Onustus. 
In the Jurassic rocks of this country the genus is first noted in the Upper 
Lias, where fairly characteristic specimens of 0. heliacus, d’Orb., are quoted from 
more than one locality, and a similar form occurs in the Lias-Oolite sands of 
Nibley, in Gloucestershire. 
A small and interesting form, near to O. heliacus, occurs sparingly in the 
Opalinus-zone; whilst the Murchisonx-zone is characterised by a group of much 
larger formss which it is convenient to focus for the most part under Onustus 
pyramidatus, Phil. In the Upper Division of the Inferior Oolite Onustus 
ornatissimus, @Orb., is far from uncommon at Burton Bradstock, and a variety of 
this has been noted by Morris from the Lincolnshire Limestone. The genus is 
but poorly represented in the Inferior Oolite of the Cotteswolds, and has not been 
recorded, to my knowledge, from the Great Oolite of Minchinhampton. The French 
authors have described one or two species from the Bathonian and Callovian of 
France, and Lycett described Onustus Burtonensis from the Forest-Marble of 
Burton Bradstock. 
t «Bull. Soc. Linn. Norm..,’ voi. v. 
2 ‘Suppl. to Great Oolite Mollusca,’ p. 103. 
42 
