SPIRIFERA. 31 



cherty Carboniferous limestone, in which the spirals were beautifully preserved, while the 

 internal casts exhibit the muscular and ovarian impressions (figs. 26, 27). It is also 

 common, in this shape, in a yellow sandstone near Richmond, in Yorkshire, as well as 

 Kendal. It abounds in the dark Carboniferous limestone of Lowick, and at Dent, in 

 Yorkshire. It is not rare in Scotland, it is found at Courland, near Dalkeith; at 

 Dryden ; and Braidvvood, near Carluke j and is also mentioned from several Irish 

 localities. 



Spirifera bisulcata, Sowerby. Plate IV, fig. 1 ? Plate V, fig. 1 ; Plate VI, figs. 1 — 19 ; 



Plate VII, fig. 4. 



Spirifer bisulcatus, J. de C. Sowerby. Min. Con., tab. 492, figs. 1 and 2, 1825. 



— trigonalis, Sowerby (not Martin). Min. Con., tab. 265, figs. 2 and 3, 



1820. 



— bisulcatus, Davreux. Const. Geog. de la Province de Liege, p. 272, pi. vii, 



fig. 3, 1831. 



— — De Koninck, Animaux fossiles de la Belgique, pi. xiv, fig. 4, 



1843. 

 Phillips. Geol. of Yorkshire, vol. ii, pi. ix, fig. 14, 1836. 



— semicircularis, Phillips. Ibid., pi. ix, figs. 15, 16. 



— calcarata, M'Coy (not Sow.) Synopsis of the Carb. Foss. of Ireland, 



p. 130, pi. xxi, fig. 3. 



— trigonalis, var. bisulcata and semicircularis, M'Coy. British Palaeo- 



zoic Fossils, p. 424, 

 1855. 



Spec. Char. Semicircular or sub-rhomboidal, commonly wider than long, with valves 

 almost equally convex. The hinge-line is, in general, longer than the greatest width of 

 the shell, the cardinal extremities being rounded, or forming angles of variable projection. 

 Area moderately wide, divided by a triangular fissure, which is closed in part by a pseudo- 

 deltidium. Beaks incurved, and at times considerably approximate. The sinus presents 

 a moderate depth ; the mesial fold, regularly rounded, is not much elevated, nor does it 

 project to any extent beyond the level of the lateral margins. Each valve is ornamented 

 by from thirty to forty obtusely rounded ribs ; these are simple, and rarely bifurcated, 

 but increase occasionally by intercalations at various distances from the beaks. The ribs 

 on the mesial fold are arranged into three groups, separated by sulci of greater depth. 

 Dimensions very variable — 



Length 15^, width 18, depth 13 lines (Sowerby's type). 



„ 24, „ 41, „ 20 lines (a very large individual). 



Obs. This common Spirifer is so variable in its general shape, as well as in the 

 number and width of its ribs, that it is not always easily separated from certain excep- 



