270 BRITISH SILURIAN BRACHIOPODA. 



somewhat similar in shape to the one which in Athyris concenirica and some other 

 species is attached to the small foramen under the hinge-plate. In the internal casts of 

 the ventral valve (figs. 4 and 5) this small tube seems to have originated just above the 

 adductor, and to have curved upwards as represented in fig. 5. Its whole extent and 

 position is well seen in some specimens, although only in the shape of a cast. I have 

 never noticed a similar appendage in any species of Orthis, but indications of its 

 existence have sometimes been noticed in certain species of Spirifer. The internal cast 

 of the dorsal valve (fig. 6) is very remarkable, distinguishing the shell at once from 

 any other with which I ain at present acquainted. Mr. Salter also appears to have been 

 very uncertain as to the genus to which the shell should be referred, for at page 

 21 1 of the second edition of ' Siluria ' it is considered to be a StrojoJwmena, and at page 544 

 an Orthis. The arrangement of the muscular scars, and the shape of the cardinal process 

 remove it, I think, from Orthis. 



Position and Locality. The vertical range given in ' Siluria' is the Llandeilo and 

 Caradoc. In the Llandeilo Mags it has been found at Garn Arenig (Survey Mus.). In 

 the Caradoc it is very abundant in the shape of external and internal casts at Llanfyllin, 

 Pont Rhiwedog, Gaerfowr and Moely Garth, West Pool, Garnedd Uchaf, Bala. Mr. 

 Salter and Prof. M'Coy quote it likewise from Dolwyddelan, south-west of Pwllheli, 

 Penmachno, Caernarvonshire ; Cerrig-y-Druidion, Denbigshire ; Llanwddyn, Alt -y- Anker, 

 Meifod, Welshpool, Montgomeryshire ; east, west, and south-east of Bala Lake, Dinas 

 Mow^ddwy, Llanrhaidr, Tan-y-Groes south of Bala, schists of Belli Grin, Bala, Merioneth- 

 shire ; schists of Beaver's Grove, Bettws-y-Coed, North Wales ; schists of Rhiwargor, &c. 



In Ireland it occurs in the Caradoc of Carrichadaggan, County Wexford ; at the 

 Chair of Kildare, &c. I am not acquainted with this shell either from Scotland or 

 the Continent. 



Orthis tricenaria, Conrad (?). PI. XXXVIII, fig. 28. 



Oethis tkicenaeia, Conrad. Proc. Acad. Nat. Sciences, vol. i, p. 333, 1843. 



— —J. Hall. Pal. New York, vol. i, p. 121, pi. xxxii, fig. 8, 1847. 



— — Etlieridge. Catal. Foss. Mus. Prac. Geol., p. 8, 1865. 



— — Salter. Siluria, 4th ed., p. 527, 1867. 



Spec. Char. Almost circular, or semi-oval, about as wide as broad; hinge-line 

 straight, nearly equal to the greatest width of shell, with angular cardinal extremities ; 

 sides and front rounded. Ventral valve regularly convex; beak incurved, but not 

 produced ; area wide ; foramen triangular, open ; surface of valves marked with about 

 thirty-six rounded simple ribs, with interspaces between them of equal width. 

 Length 6^, width 6^ lines. 



