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, 0. pyramidatus, 

 Phil., appears deprived of all its costae, 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 

 0. pyramidatus, Phil., where the basal periphery is almost simple and the base 

 nearly smooth. To this group belong 0. lamellosiis, d'Orb., and 0. acuminatus, 

 sp. n. (PI. XXVII, fig. 3), in the Inferior Oolite; 0. liasinus, E. Desl., of the 

 Middle Lias of May ; and 0. papyraceus, Heb. and Desl., from the Callovian of 

 Montreuil-Bellay. (2) The second group is represented by 0. ornatissimus, d'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 strige. This group includes 

 0. heliacus, d'Orb., of the Upper Lias, and the var. opalina (PI. XXVII, fig. 4). 

 In 0. Heberti, Laube, the characters of the two groups seem to be somewhat 

 blended. 



259. Onustus pyramidatus, Phillips, 1829. Plate XXVII, figs. 1 and 2. 



1829, 1835. Teochus pybamidatus, Phillips. Geol. Yorks., pt. 1, pi. xi, fig. 22. 



1884. Onustus pybamidatus, Phil., sp. Hudleston, Geol. Mag., dec. 3, vol. i, 



p. 294, pi. ix, figs. 2 and 3. 

 See also for this group of Onustus — 

 Teochus lamellosus, d'Orbigny. Terr. Jur., vol. ii, p. 270, pi. cccxi, 



figs. 11—13. 

 Onustus liasinus, Eugene Deslong champs. Bull. Soc. Linn. Norm., vol. v> 



pi. x, fig. 10. 



— papteaceus, Heb. and Desl. Op. et vol. cit., pi. ix, fig. 3. 



— oenatissimus, Rudl., non d'Orb. Geol. Mag., dec. 3, vol. i, 



p. 293, pi. ix, fig. 1. 



Bibliography, 8fc. — The original Trochus 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. 



