ORTHID.E. 273 



impure Bala Limestone of Revenstone Dale and Ash Gill, Westmoreland ; Plain-y-Cwm, 

 west of Nantyr, Glyn Ceiriog, south of Llangollen, Denbighshire. Li Coniston Limestone 

 at Helm Gill, in the lake district, &c. 



This shell is very abundant in the LTpper and Lower Silurian rocks of many countries 

 abroad. 



It occurs in the neighbourhood of St. Petersburgh, Russia, and of Reval ; in the 

 Lower Silurian rocks of Bodahamn, in the Island of Oeland ; at Malmokalven, in 

 Scandinavia ; at Wisby, in the Island of Gothland, &c. 



In America Prof. Hall states that it occurs " in numerous and widely separated 

 localities of the Trenton Limestone, being confined to rocks of this period in the United 

 States, and unknown in New York above the Utica Slate, though this line of demarcation 

 cannot be recognised in the west. Within the State of New York it is found at Middleville, 

 Trenton Palls, Herkimer, Jacksonsburgh, and in other localities in the Mohawk Valley. 

 It is also found at Turin, Watertown, and other places towards Lake Ontario, and is 

 likewise known at several places in the Champlain valley (Tennessee) ; at Cincinnati and 

 Oxford (Ohio) ; at Madison (Indiana) ; at ]\Iineral Point (Wisconsin), and on the north- 

 west shore of Lake Michigan, and towards Green Bay. It likewise occurs in several 

 localities in Canada, east and west, showing a geographical distribution equal to the 

 extent of the Lower Silurian strata of America." 



Orthis insulaeis, JD'Eichcald. PI. XXXVII, figs. 8 — 15. 



Tekebbatula insl'Laris, D'Eic/iw. Urwelt Russl., vol. ii, p. 49, pi. ii, figs. G a, b, c, 



1842. 

 SpiiiiFER — Be Verneuil. Geol. of Russia, vol. ii, p. 149, pi. viii, figs. 7 



a, b, c, 1845. 

 Orthis galea, M'Coy. Syn. Sil. Foss. of Ireland, p. 30, pi. iii, fig. 12, 1840. 



— insularis, Phillips and Salter. Mem. Geol. Survey, vol. ii, p. 289, 1848. 



— — Morris. Catalogue, p. 140, 1854. 



Spirifeu — D'Eichiv. Lethaea Rossica, Ancienne Periode, vol. i, p. 697, 1859. 



Orthis — Sailer. Mem. Geol. Survey, vol. iii, pp. 25(5 and 268, 1866. 



— — Id. Siluria, 4tli ed., p. 526, 1867. 



— — Bigshy. Thesaurus Siluricus, p. 105, 1868. 



S^ec. Char. Transversely oval or globulous, slightly indented in front, lateral 

 margins rounded ; hinge-line rather more than half the width of shell. Ventral valve 

 very gently convex, with a deep longitudinal sinus, commencing at the extremity of the 

 beak and gradually widening as it nears the front ; area triangular, about one fourth as 

 high as long ; fissure open ; beak small, slightly incurved, scarcely projecting. Dorsal 

 valve very gibbous, much deeper than the opposite one, the umbonal beak being large, 



35 



