ONUSTUS. 325 



Genus — Onustus, Humphrey, 1797. 



We owe to the late Eugene 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 I860, 1 and pointed out that in the Jurassic rocks of France the 

 genus had been noted in the Middle Lias, the Upper Lias, the Inferior Oolite, 

 the Great Oolite, and the Callovian. Lycett in 1863 2 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, ivhieh overhang and conceal the suture ; the lower 

 border of the last lohorl is produced horizontally ; surface irregularly striated. 



TJie 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 Lycett. 



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 0. heliacus, occurs sparingly in the 

 Opalinus-zone ; whilst the Murchisonse-zone is characterised by a group of much 

 larger forms, which it is convenient to focus for the most part under Onustus 

 pyramidatus, Phil. In the Upper Division of the Inferior Oolite Onustus 

 ornatissimus, d'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. 



1 'Bull. Soc. Linn. Norm.,' vol. v. 



2 ' Suppl. to Great Oolite Mollusca,' p. 103. 



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