218 CARBONIFEROUS LAMELLIBRANCHIATA. 



obliquely diagonal ridge passes from the extreme apex of the umbo downwards 

 and backwards to the posterior-inferior angle, becoming obsolete in its lower 

 third; above and posterior to this ridge the valve is rapidly compressed, so that 

 the posterior slope becomes slightly concave. 



Interior. — The anterior adductor muscle-scar is situated immediately within 

 the antero-superior angle of the valve, and is oval ; that of the posterior 

 adductor, somewhat larger, is situated near the upper margin of the valve, some 

 little distance from the posterior border. The pallial line is entire and well 

 marked, submarginal. Au internal ridge extends obliquely from the umbonal 

 cavity to the postero-inferior angle. 



The Hinge. — Bight valve : There is a small and almost obsolete anterior tooth, 

 being simply a thickening of the edge of the valve, directed obliquely downwards 

 and forwards. The cardinal tooth is thin above and swollen below, inclined 

 forwards and downwards. The posterior tooth is long, separated from the 

 margin by a shallow elongate furrow. On either side of the cardinal tooth is a 

 deep, obliquely delta-shaped hollow. 



Left valve : The anterior tooth is small, not elongate, projecting backwards. 

 The cardinal tooth is prominent, massive, thicker below and above, and inclined 

 obliquely backwards and downwards, obscurely bifurcate, or with its ventral 

 margin concave below. The posterior lateral tooth is elongate and almost 

 horizontal. When the valves are in apposition the teeth of the right valve are 

 severally anterior to the corresponding teeth of the left. 



Exterior. — The surface is ornamented with regular, close, fine, concentric 

 striae and lines of growth, which become more rugose towards the lower margin. 



Dimensions. — Fig. G, PI. XVI, measures — 



Antero-posteriorly . . . .33 mm. 



Dorso-ventrally . . . .25 mm. 



Elevation of valve .... 10 mm. 



Localities. — Scotland : Fifeshire, Ardross Limestone, Ardross, base of Lower 

 Carboniferous Limestone group. Cambo Ness, Kingsbarns ; Limestone No. 5, 

 Randerstone. In a 2-foot limestone east of the Target, Billowness, between 

 Pittenween and Anstruther, Fifeshire (zone 12 of Mr. Kirkby) ; Wardie Shale 

 group, Woodhall, Water of Leith, — all in the Calciferous Sandstone series of 

 Scotland. In a bed of impure limestone above the Cooper's-eye Coal, Scre- 

 merston, Northumberland. 



Observations. — This species has been long recognised as occurring in the Lower 

 Carboniferous beds of the east of Scotland, and has been alluded to by several 

 authors. The fullest description of the species is that by R. Etheridge, jun., who 

 gave it the specific name Salteri. The Rev. T. Brown had prior to this referred 

 the shell, on the authority of Mr. Salter, correctly to the genus Schizodus {op. sup. 



