18 ORDOVICIAN AND SILURIAN BELLEROPHONTACEA. 



regarding as of little importance the umbilical ridge and the lateral notch near 

 the mouth mentioned by Koken^ in his original definition of the latter genus. 



1. Sinuitopsis congruens, sp. nov. Plate III, fig. 15. 



Specific Characters. — Shell high, coiled in rather loose spiral, composed 

 of about three rounded whorls in contact but scarcely overlapping, rapidly 

 increasing in size, subcircular to elliptical in section, rather higher than wide ; 

 dorsum rounded, not compressed. Umbilicus open, exposing inner whorls, with 

 its centre situated at about one-fourth the height of shell ; umbilical edges rounded. 

 Mouth not expanded, with deep U-shaped sinus in outer lip, followed by obscure 

 broad median band of same width as sinus, dying out at about half the length of 

 outer whorl. Surface of shell with a few transverse growth-lines and stronger ridges 

 close to apertural margin, curving back to meet band on dorsum at about 10° — 15°. 



Dimensions. — Height of shell, 8'0 mm.; width of mouth, 5*0 mm. 



Horizon. — Lower Ordovician : Balclatchie Group. 



Localitij. — Balclatchie, Girvan. 



Bemarls. — The open umbilicus, the rapid rate of increase in the size of the 

 whorls, the small degree of overlapping, and the band-like structure on the last 

 half of the outer whorl resemble Sinuitopsis neglecta (Barr),- but the deeper 

 U-shaped sinus in the lip is more like that in Tenmodiscus 2^^(itl/notus, Perner.^ 

 As above stated there is some doubt as to the genus to which the Girvan 

 species should be referred. The holotype is in Mrs. Gray's Collection. 



Genus OXYDISCUS, Koken. 



Generic Characters. — Shell strongly compressed, disciform ; whorls embracing 

 very little, expanding gradually to the aperture, sharply keeled, lanceolate or 

 subtrigonal in cross-section ; dorsal lip with deep V-shaped sinus ; no slit-band. 

 Umbilicus large, open, exposing most of the whorls. Surface with transverse 

 growth-lines bending backward on the keel. 



The definition given by Koken'^ is inadequate, and he mentions the occurrence 

 of traces of a slit-band which seems certainly absent in the British Lower 

 Palaeozoic shells here referred to this genus. Shells with a similar external 

 appearance, but possessing a slit-band, are now put in the genus Zonidiscus, Spitz 

 (see below). Ulricli and Scofield's^ definition likewise seems to include species 



' Koken, ' Gastrop. Bait. TJntersilurs,' p. 129. 



2 Perner, op. cit., p. 68, pi. Ixxxv, fig. 11 ; pi. Ixxxviii, tigs. 28—30, 38—40 ; pi. xcvii, figs. 39—41 ; 

 text-figs. 42, 4G. 



3 Ibid., p. 7(3, text-fig. 51. 



* Kokeu, 'Neues Jalirb. f. Min.,' siippl. vol. vi (1889), pp. 390, 392, pi. xii, fig. 4; id. 'Die 

 Leitfossilien ' (1896), pp. 100, 393. 



5 TJlrich and Scofield, op. cit., pp. 852, 912, 



