150 SUPPLEMENT TO THE 



with two or three ribs, and five or six strong ones on each side of the Lateral portions of the 

 valve ; beak moderately incurved, leaving between the beak, ridges, and the hinge-line a 

 well-defined narrow area ; foramen circular, and situated under the extremity of the 

 beak. 



Length 7, width 9, depth 7 lines. 



Obs. — For a long time I felt undecided whether this Bhynclionella was more than a 

 variety of R. horealis, to which it is closely allied. It differs, however, from Schlotheim's 

 species in being more transversely oval, and especially by its long and nearly straight 

 hinge-line and area. 



Hh. suh-horealis was found by Mrs. R. Gray in the Upper Llandeilo at Craighead, 

 Girvan, Ayrshire, Bh. horealis proper is an Upper-Silurian fossil. 



34. Rhynchonella bideistata, Ilisinger^ sp. Dav., Sil. Mon., PI. XXL fig. 22 ; Sil. 



Sup., PI. X, fig. 3, 3 a, h. 



Terebr-^tula bidentata, His. Vet. Acad. Hand., p. 323, tab. viii, fig. 5, 1826. 

 Anteckningar Physik ocli Geognosi, vol. iv, tab. 

 vii,fig. 5, 1828. 



— — Balmun. Vet. Acad. Hand!., p. 142, tab. vi, fig. 5, 1828. 



— — Hisinger. Icon. Petref. Svec, tab. xxiii, fig. 7, 1837. 



— BOREALis, Quenstedt. Handbuch der Petrefactenkunde, p. 35, fig. 



36 a, b, 1851. 



— — var. INDENTATA, Dav. Sil. Men., p. 1/4, pi. xxi, fig. 22, 



1866. 



Shell small, sub-pentagonal, wider than long ; dorsal valve convex, with eight strong 

 radiating ribs, of which the two central ones are close together ; fold large, slopes wide 

 and flat ; the fold occupies one third of the breadth of the shell ; three large ribs on each 

 of the lateral portions of the valve ; ventral valve moderately convex, sinus large and 

 deep, divided longitudinally by a narrow rib ; surface of the valves and ribs intersected 

 by numerous equidistant, scabrose, concentric, projecting ridges ; beak tapering, very 

 slightly incurved, with a circular foramen under, or slightly truncating the extremity of 

 the beak, and separated from the hinge-line by a small deltidium. 



A large specimen measured : length 4, width 4|^, depth 3 lines ; but most specimens 

 do not exceed 2| lines in length by 3 in width and \ in depth. 



Ohs. — This is apparently always a very small shell, never attaining anything like the 

 dimensions of either Rh. horealis or suh-horealis, or R. diodonta. Hisinger and Dalman's 

 descriptions and figures of this shell agree in every respect with the specimens we find in 

 England. " Terebratula pusilla testa triangulari, apice rotundata, radiatim plicata, 

 Iscvi ; yalvfle majoris sinu unicostato apice bifido ; in Gottlandia ad Djupviken " 



