246 CATALOGUE OF THE FOSSILS OF THE PERMIAN SYSTEM 
In Magnesian-limestone ; not uncommon. Humbleton, Tun- 
stall, Silksworth, Cornforth ; Whitley Quarries. 
In this species the margins are more nearly parallel, and the 
posterior end is much wider and more truncated than in the JA. 
obscurus. ‘The anterior is rather convex, and very much rounded. 
The beaks are prominent, and the surface of the valves is slight- 
ly waved and ornamented with numerous concentric acute 
ridges, which are most conspicuous on the anterior extremity. 
2. A. opscurus, Sow. 
Axinus obscurus, Sow. Min. Conch. tab. 314. 
In Magnesian-limestone ; not common. Marsden, Byers’ 
Quarry, Roker. 
The specimens which are referred to this species occur in seve- 
ral of the thin beds of limestone on the coast between South 
Shields and Sunderland. They are generally much smaller than 
those which occur in Yorkshire. 
Famity. Arcana, Gray. 
21. ARCA, Linn. 
1. A. LorrusIANa, 7. s. 
Shell transversely elongated, sub-rectangular ; anterior, 
short, tumid ; posterior, elongated, often slightly compressed, 
obliquely truncated ; beaks prominent, approximating ; sur- 
face strongly ribbed ; ribs, numerous, strong, rarely bi- 
furcated, granulated on the anterior and posterior ; hinge- 
line, with four or five oblique teeth on the anterior, and 
two or three parallel teeth on the posterior side. 
In Magnesian-limestone ; not common. Tunstall, Humbleton. 
The general form of this shell is slightly rectangular owing to 
the free margins being almost parallel to the hinge-line, and to 
the posterior being much elongated. The anterior in some spe- 
cimens is very tumid, in others slightly compressed, compara- 
tively short and rounded. The posterior is very much elonga- 
ted, widened, flattened, and obliquely truncated. The surface is 
covered with numerous strong radiating ribs, which are covered 
with distinct, close granulations on the posterior and anterior 
parts of the shell. The beaks are large and placed near to the an- 
