SCIIIZODUS AXINIFORMIS. 221 



and posterior margin. The pallial line is entire and remote from the margin. 

 The hinge is apparently normal, but I have not been able to expose the whole of 

 it in any one specimen. In casts, several obscure radiating sulci are to be seen 

 on and anterior to the posterior umbonal ridge. 



The ligament is external, small, and short. 



Exterior. — The surface is adorned with regular, concentric, fine lines of growth, 

 which are crowded in front, but soon become separated and equidistant, dividing 

 the surface of the shell into a series of regular, equal, concentric bands, which are 

 bent sharply at an acute angle along the line of the dorsal ridge ; from this point 

 they become linear, and terminate on the upper margin of the valve. Occasion- 

 ally one of the concentric lines is accentuated, and becomes imbricate. 



Dimensions. — Fig. 11, PI. XVI, measures — 



Antero-posteriorly . . . .06 mm. 



Dorso-ventrally . . . .46 mm. 



From side to side . . . .36 mm. 



Localities. — England : the Redesdale Ironstone, Lower Carboniferous, North- 

 umberland ; Pennystone Ironstone, Coalbrookdale Coal-measures; Rosser Vein, 

 Cwm Bryn ddu, South Wales Coal-measures. Scotland : in Calcareous Sandstone, 

 Garngad Road, Glasgow; Upper Limestone series. 



Observations. — The original specimen on which Phillips founded his species 

 Isocardia? axiniformis is stated to have come from Northumberland, though the 

 exact horizon is not noted. The species occurs in abundance in the Redesdale 

 Ironstone Shales. I am unable to differentiate the species described by Phillips 

 from the Donax ? sulcatus of J. de C. Sowerby, which I have placed as a synonym. 

 King seems to have come to the same conclusion, for he states ('Monograph of 

 Permian Fossils,' p. 185), "Mr. Prestwich kindly allowed me to examine the 

 originals of Mr. Sowerby's species, and I find from my memoranda made at the 

 time that Donax ? sulcatus . . . is the Isocardia axiniformis of Phillips." I, too, 

 have had the same privilege (the Coalbrookdale shells are now in the collection of 

 the British Museum [Nat. Hist.], South Kensington), and have come to the same 

 conclusion. The Coalbrookdale specimens are always in the form of casts, but 

 fortunately a few testiferous examples occur at Redesdale. In the latter locality 

 the shell reaches a much larger size than at Coalbrookdale, and examples occur in 

 all stages of growth, which show that with increasing age the posterior end 

 becomes more pointed and narrower, though there is some variation in contour to 

 be seen in certain individuals. 



M'Coy {op. cit., p. 56) refers the Isocardia axiniformis of Phillips to Gardio- 

 morpha, but he says of it, " This species much resembles an Axinus in its depressed 

 hatchet-like form, and obliquely truncated compressed posterior side; the large in- 

 curved beaks, however, approximate it more to Isocardia or Cardiomorpha." This 



