6 ORDOVICIAN AND SILURIAN BELLEROPHONTACEA. 



type. Three specimens in the Sedgwick Museum show similar variation. With 

 regard to affinities, the flattened dorsum recalls the species Sinuites planodorsatus, 

 Ulr.,^ from Kentucky, but the shape of the sides, height of the outer whorl, and 

 depth of the sinus forbid us considering it identical. The apertural curve is 

 in no specimen perfectly preserved. 



2. Sinuites balclatchiensis, sp. nov. Plate I, figs. 4 — 7. 



Specific Characters. — Shell subglobose, rounded. Outer whorl completely 

 embracing inner whorls, increasing slightly in height and more rapidly in 

 width to moutli, with dorsum high, rounded, strongly arched, but becoming 

 obtusely angulated towards mouth. Mouth about one and a half times as wide 

 as high, slightly expanded at sides, with rounded, U-shaped sinus in outer lip and 

 rounded apertural lobes strongly arched forwards. Apertural curve rounded, not 

 angulated. Umbilicus closed; situated at about half the height of the shell or 

 })elow the middle. 



Surface with very faint transverse strige near lip, but elsewhere smooth. Interior 

 of shell finely granulose, generally without any marginal thickening of lips but 

 with one transverse internal thickening on dorsum some distance behind mouth. 

 Shell-substance thin. 



Dimensions. — Height of shell, 18'0 mm. ; height of outer whorl above umbili- 

 cus, ll'O mm.; height of outer whorl below umbilicus, 7'0 mm. ; width of outer 

 whorl above umbilicus, 14'5 mm. ; width of outer whorl below umbilicus, 9'5 mm. 



Horizon. — Lower Ordovician : Balclatchie Group. 



Localitij. — Balclatchie, Girvan . 



Remarks. — The relations of this species to the typical S. bilubatus (Sowerby) ^ 

 are close, but the Girvan form differs by having a more sharply and narrowly 

 arched back, parabolic rather than semi-elliptical in cross-section, and a more 

 rapid increase in the width of the whorls towards the mouth. As mentioned 

 below, the name hllohatus has been applied in a very loose and indefinite 

 manner, and several species have been included by British palseontologists under 

 this specific designation, while its varied usage by foreign geologists has still 

 further increased the confusion. 



In all of the specimens from Balclatchie in Mrs. Gray's collection the shell is 

 very thin, and has a shining, black, corneous appearance, which seems to be due to 

 its natural and original character, and not to secondary changes or to the state or 

 method of preservation. 



The holotype is in Mrs. Gray's collection. 



1 Ulrich & Scofield, op. cit., p. 871, pi. Ixiii, figs. 31—35. 



- Sowerby in Murcliison's ' Silur. Sjst.,' p. 643, pi. xix, fig. 13. 



