ALLORISMA SULCATA. 423 



posterior scar, pear-shaped and large, is placed immediately below the hinge-line, 

 remote from the posterior end. A few specimens show indications of a deep 

 pallial sinus. The inner surface of the shell had concentric grooves and ridges, 

 much as on the external surface, but not so sharply cut. 



Exterio)'. — The surface is ornamented with regular, distinct, simple grooves 

 and ridges, which follow the contour of the valve, the ridges and sulci being of 

 almost equal width. 



In well-preserved specimens the shell is covered by radiating rows of minute 

 tubercles, much more apparent on the posterior moiety of the valve and the dorsal 

 slope. Shell very thin. 



Dimensions. — PI. XLVIT, fig, 8, a specimen from Beith, Ayrshire, measures — 

 Antero-posteriorly . . . .62 mm. 



Dorso-ventrally . . . .30 mm. 



From side to side . . . .22 mm. 



Localities. — England : the Redesdale Ironstone, and Lowick, Northumber- 

 land ; under the Farewell Rock of Glan Rhymney and Beaufort, South Wales ; 

 also 500 feet below the third Millstone Grit, Congleton Edge, Cheshire. Scot- 

 land : the Upper Limestone Series at Garngad Road, Glasgow ; the Lower 

 Limestone Series of Beith. Den Quarry, Kincaple, near St. Andrews ; Newtown 

 Quarry, Knockhill ; Billowness, zone 8 of Mr. Kirkby, Fife, Calciferous Sandstone 

 series ; Shore under Round Tower, St. Monans, Fife, Carboniferous Limestone 

 series. Ireland : in Shale, co. Fermanagh. 



Observations. — This species has been surrounded with much obscurity, chiefly 

 due to the fact that authors have not recognised the essential differences 

 between Allorisma sulcata and Edmondia sulcata, so well pointed out by King in 

 his Monograph on the Permian Fossils {op. cit., p. 163). Referring to this point 

 (see above, p. 319) in my remarks on Edmondia sulcata, I pointed out how 

 completely the two species differ from each other in every detail, though to the 

 casual observer they may seem to be very similar. The hinge-plates are essen- 

 tially different ; Allorisma sulcata not possessing the ossicle present beneath the 

 umbo in all species of Edmondia, and showing as a slit in casts of the interior. 

 A. sulcata has a well-marked lunule, and a specially well-developed escutcheon, 

 both of which are absent in E. sulcata. In A. sulcata the umbones are not well 

 marked off from the anterior part of the valve, because of the obliteration of the 

 antero-superior angle of the shell, and the depressed lunule ; moreover the 

 anterior border seems to pass up into the umbo without any break, but this is not 

 the case in E. sulcata. In A. sulcata the grooves and sulci are simple and 

 unbroken, passing from before backwards without a break. In E. sulcata the 

 ridges are more numerous in front, two coalescing to form a thicker ridge 

 posteriorly and often compound. Further, A. sulcata has a well-marked com- 



