326 GASTEROPODA OF THE INFERIOR OOLITE. 
There can be little doubt that these shells have their ornamentation con- 
siderably modified by the accidents of preservation and mineralisation, so that 
appearances are not altogether to be trusted. This is especially the case in the 
Dogger, where it sometimes happens that the characteristic species, O. pyramidatus, 
Phil., appears deprived of all its costz, or is otherwise so modified as to invite 
distinct recognition. Hence my hesitation to deal with forms which undoubtedly 
present considerable variety, and which under other conditions I should scarcely 
hesitate to regard as fairly distinct species. Roughly speaking, the Jurassic species 
referred to Onustus may be divided into two groups, viz. (1) that represented by 
O. pyramidatus, Phil., where the basal periphery is almost simple and the base 
nearly smooth. To this group belong O. lamellosus, d’Orb., and O. acwminatus, 
sp. n. (Pl. XXVII, fig. 3), in the Inferior Oolite; O. liasinus, E. Desl., of the 
Middle Lias of May; and O. papyraceus, Héb. and Desl., from the Callovian of 
Montreuil-Bellay. (2) The second group is represented by O. ornatissimus, @Orb., 
where the basal periphery is more or less serrated, whilst in the majority of cases 
the base itself is more or less marked by spiral striz. This group includes 
O. heliacus, @’Orb., of the Upper Lias, and the var. opalina (Pl. XXVII, fig. 4). 
In O. Heberti, Laube, the characters of the two groups seem to be somewhat 
blended. 
259. Onustus pyramipatus, Phillips, 1829. Plate XXVII, figs. 1 and 2. 
1829, 1835. Trocuus pyramipatus, Phillips. Geol. Yorks., pt. 1, pl. xi, fig. 22. 
1884. Onustus pyRAMIDaATuS, Phil., sp. Hudleston, Geol. Mag., dec. 3, vol. i, 
p- 294, pl. ix, figs. 2 and 3. 
See also for this group of Onustus— 
TRocHUS LAMELLOsUS, d’Orbigny. Terr. Jur., vol. ii, p. 270, pl. ecexi, 
figs. 1I—13. 
Onusrus Liasitnus, Hugéne Deslongchamps. Bull. Soe. Linn. Norm., vol. v, 
pl. x, fig. 10. 
—  papyraceus, Héb. and Desi. Op. et vol. cit., pl. ix, fig. 3. 
—  ornarisstmus, Hudl., non d’Orb. Geol. Mag., dec. 3, vol. i, 
p- 298, pl. ix, fig. 1. 
Bibliography, Sc.—The original Trochws pyramidatus, now preserved in the 
York Museum, does not very much resemble the figure by Phillips in the 
‘Geology of Yorkshire,’ which seems to represent a more costated shell. I think, 
however, the specimen may have suffered from scaling, to which fossils from the 
Dogger are liable. 
