DURANIA. 421 



1854. Eadiolites Mortoni, J. Morris. Cat. Brit. Toss., ed. 2, p. 160. 

 ? — — sp., Morris. Ibid., j). 160. 



1000. BiBADioLiTES MoRTONi, H. Douvillr. Bull. Soc. gcol. de Frauce, ser. 3, 



vol. xxviii, p. 230. 



1904. — — BouviUe. Ibid., ser. 4, vol. iv, p. 174. 



1909. Sauvagesia Mortoni, A. Toucas. Classific. et ]<ivoIut. des Radiolitides 



(Mi'iu. Soc. gcol. de France, Palcont., 

 vol. xvii), p. 92, fig. 59. 



1911. IvADioLiTES Mortoni, F. Francke. Zeitsclir. d. deutsch. geol. Gesellsch., 



vol.lxiii, Mouatsber., p. 357. 



Non 1865. — Mortoni, iS'. 1'. Woochranl. Quart. Jouru. Gcol. Soc, vol. xi, 



p. 59, pi. v, figs. 1, 2. 



— 1866. — — K. A. Zittel. Bivalv. d. Gosaugeb. (Deukschr. d. U. 



Akad. Wissenscli. Wien, Math.-uat. 

 Cl., vol. XXV, pt. ii), p. 72, pi. xxv, 

 figs. 1—3. 



— 1903. — — V. Hilber. Jalirb. d. k. gool. Reichsaust., vol. lii, 



(1902), p. 282. 



Description. — Lower valve at first conical, afterwards becoming elongate and 

 cylindro-conical, nearly straiglit or slightly arched. Longitudinal ribs strong, 

 angular, sometimes in groups of two or three, crossed by growth-lines. Bands 

 concave, with fine ribs ; the interband convex, usually with strong ribs. Radiating 

 grooves bifurcate once, twice or more in jjassing from the inner to the outer 

 margin. The cellular structure varies in coarseness in different specimens, 

 and sometimes becomes rather finer towards the outer margin. Upper valve not 

 known. 



AMnities. — One of the specimens figured l)y Sowerby (Dixon, fig. o) is distin- 

 guislied from the others by its concave interband with fine ribs and by the narrower 

 siphonnl bands ; it probably belongs to another species. 



Bemarks. — Fragments which probably belong to this species are moderately 

 common in the Lower Chalk, and in the Cambridge Greensand,' but good specimens 

 are rare. Some examples are of considerable size, the largest being a specimen 

 from the Cambridge Gi-reeusand, in which the lower valve has a depth of 21 inches. 



Ti/pc.-i. — Mantell's specimens cannot be traced. The examples figured by 

 Sowerby (in Dixon, figs. 1 — -h), from the zone of Hohister suhijlobosuti, near Lewes 

 (probably Glynde), and the specimen figured by Hudson from Lewes, are in the 

 British Museum. 



DlstnbHtion.—Ca.mhrk\ge Greensand (base of Chalk Marl). Probably Chalk 



' Another species of Durania is represented in the Cambridge Greensand by a form with broad, 

 slightly concave bauds with fine ribs, and a narrow, strongly concave interband with coarse ribs. The 

 bands and iutorband rosoiulilo those of some species of Sauvagesia (Toucas, 1909, pi. xvii, figs. 1,3, 6). 



55 



