OXYDJSCUIS ACUTIIS. 19 



possessing a slit-band. Tlio (n-i<.-inal type chosen preferably by Koken for the 

 genus, is the Middle Devonian shell 0. imif((for, Koken, from the Eifel, instead 

 of B. currnhicatus, Conrad, of the Corniferons Limestone, which is the ty{)c of 

 Meek's distinct genns Tropidodiscus,^ al)ont wliich there has been unwarranted 

 confusion. Perner's^ subgenus of O.t'i/disniii, named Gi/rtodiscns, is not here 

 regarded as well established. 



1. Oxydiscus acutus (Sowerby). Plate IV, figs. 1, 2. 



1839. Bellerojjhon acutus, Sowerby in Miircbison's Silurian System, p. 643, pi. .\ix, fiJ,^ 14. 



1848. Bellerophon acutus, Ferussac et D'Orbigny, Hist. Nat. Ceph., vol. i, p. 208, ])!. viii, figs. 10 11. 



1852. Bellerophon carinatus, Sowerby, McCoy (pars), Syu. Brit. Pal. Foss. Woodw. Mus., fasc. ii, 

 p. 309. 



Specific Gharacfers. — Shell lenticular, composed of few whorls ; umbilicus 

 about one-third or rather less than one-third the diameter of the shell, with 

 subangular umbilical edges and short steep umbilical slope. Whorls triangular 

 in cross-section, higher than wide ; outer whorl large, increasing rather slowly in 

 size, with sharply carinated acute dorsum and more or less flattened sides. 

 Mouth oblique to vertical axis, high, narrow. Surface of shell covered with fine, 

 transverse, oblique striee, very slightly arched back. 



Dimensions. — Height (average), 10 — 40 mm.; thickness (average), 4 — 5 mm. 



Horizon. — Bala Series. 



Localities.— {I) Horderley [(391 G] ; (2) Onny River and Ticklerton [28021 

 —28024, Jermyn Street Mus.] ; (3) Twll-du. 



Eemarlcs.-—T\ie type of this species is from the " upper beds of the Caradoc 

 Sandstone " of Horderley, and Sowerby {op. cit.) described it as follows : "Com- 

 pressed, smooth, umbilicated ; whorls keel-shaped, acute : umbilicus broad ; aper- 

 ture triangular, longer [= higher] than wide. Diameter nearly half an inch, 

 width of aperture about two lines." 



The original specimen [0916] is in the condition of an internal cast, and there 

 is no sign of a slit-band on it or on any other specimens which I have seen. For 

 this reason it seems necessary to refer the species to the genus Oxydiscus rather 

 than to ZonidiscuSi though the shells bear a considerable resemblance to Z. grayi 

 and Z. shallochensis from the Girvan area. We may, however, call attention to the 

 resemblance of 0. acutus to 0. hunteri and the other species with which the latter 

 is compared. Only in one specimen [28024] have I seen the surface-ornament 

 preserved. 



McCoy {op. cit,) included 0. acutus with his B. carinatus, putting them both 

 under the name Bellerophon carinatus, Sowerby ; and consequently his specific 

 diagnosis is based on two distinct forms. 



1 Clarke, 'Palseoz. Faunas of Para' (1900), p. 40 ; id. 'Foss. Devon. Parana' (1913), pp. 390, 391. 

 ~ Perner, op. cit., p. 72. 



