Davidson — Earliest British Brachiopoda. 315 



Orthis carausii, Salter, M.S. V\. XVI. Fig. 23. 



Sub-orbicular, rather wider than long ; hinge-line the same or a 

 little less than the width of the shell. Slightly indented in front. 

 Dorsal valve gently convex with a longitudinal depression along the 

 middle. Ventral valve deeper than the opposite one ; beak small 

 incurved. Area rather narrow. Surface of valves covered with 

 about sixteen simple rounded ribs, and concave interspaces of about 

 equal breadth. Length seven, width eighth, depth three lines. 



This important species was discovered in the Lower Arenig Kocks 

 of St. David's by Mr. Hicks about the year 1864, the shell occur- 

 ing in vast abundance in the condition of internal casts and external 

 impressions in yellow sandstone at Tremanhire, and in an extraor- 

 dinary state of distortion in darker shales at Llanveran, Whitesand 

 Bay, and at Kamsey Island, St. David's. Indeed, so great is the state 

 of distortion from the effects of cleavage that one example found by 

 Mr. Homfray at Llanveran measured five lines in length by twenty- 

 seven in width, while the width of the shell prior to fossilization 

 would not liave exceeded seven or eight. In other cases the shell 

 was similarly elongated or twisted to one or other side in the most 

 irregular possible manner. 



With these few remarks I will conclude; the whole subject in 

 connection with this paper will in due time be more fully elaborated 

 in my large Silurian Monograph. 



EXPLANATION OF PLATES XV. & XVL— PLATE XV. 



Fig. 1. Lingulellaferruginea (var ovalis), Salter, from purple beds of the Harlech 

 group, St. David's. Fig. la, magnified. 

 2a, b. „ „ from Porthclais Harbour, near St. David's, from 



tbe lowest beds of the purple and red Cambrian 

 rocks and the earliest Brachiopod hitherto dis- 

 covered ; 2 and 2a, nat. size ; 2a, enlarged. 

 3-8. „ „ 3 and 4 from the 'Menevian,' St. David's ; 4 nat. size, 



after Mr. Salter's original figures ; 5 from Penmorfa, 

 Tremadoc (Upper Dolgelly group of Belt) ; 6 from 

 Gwern-y-barcud (Upper Ffestiniog) ; 7 from Ehiw- 

 felyn (Upper Dolgelly beds) ; 8 from Tremadoc or 

 Arenig group at Waun-feddu, Dolgelly. With re- 

 ference to Figs. 5 to 8, some uncertainly may still 

 prevail, although they very probably belong to L. 

 ferruginea. 

 9. „ ? specimen referred to L. Davisii by Mr. Belt from 



above Dolgelly, in his Upper Maentwrog beds. 



10-12. ,, lepis, Salter, different forms from Penmorfa church Portma- 



doc, Lower-Tremadoc. 10a, b, tvsro of Mr, Salter's 

 original figures. 



13-15. „ iJavwu, M'Coy. 13, perfect specimens from Upper Lingula 



flags near Portmadoc; 14, internal cast; 15, small 

 specimen out of shape from Gwern-y-barcud, Upper 

 Ffestiniog. 

 16. Lingula petalon. Hicks. Arenig rocks, Whitesand Bay. 



17-24. Obolella Sagittalis, Salter. A series of interiors and internal casts of both 

 valves, with figures greatly magnified, the small 

 ones being of natural size, from the Menevian rocks 

 of St. David's. Fig. 19, interior of dorsal valve; 

 20, interior of ventral valve, Mawddach Waterfall, 

 Dolgelly. 



25-57. „ Belti, Dav. 25a, 27a, and 26a, enlarged. Lower Tremadoc, 



Craig-y-dinas, North Wales, 



