OF THE COUNTIES OF NORTHUMBERLAND AND DURHAM. 240 
FAMILY. 
19. MYOCONCHA, Sowerby. * 
1. M. costata, Brown. 
Arca costata, Brown, Manch. Geol. Tr. i., pl. 6, f. 34, 35. 
Modiola costata, Geol. Russ. i., p. 224. 
In Magnesian-limestone ; common. Humbleton, Tunstall, 
Silksworth, Roker, Souter Point, Whitley. 
This species is of an oblong ovate form, with the anterior very 
slightly produced and rounded. The posterior is much elongated 
and slightly rounded. The surface is ornamented with five or 
more radiating ribs, which are slightly elevated and confined to 
the posterior portion of the shell. The surface between these 
ribs is flattened and covered with strong lines of growth. The 
hinge-margin of the right valve is furnished with a distinct, ob- 
lique tooth or callosity, which fits into a corresponding depres- 
sion in the left valve. The anterior muscle secretes a small 
ridge or bar under the beak, which leaves a groove in the cast. 
The scar of the posterior muscle is large and oval, and no sinus 
has been observed in the pallialline. It agrees very well in gene- 
ral characters with the Myoconcha of Sowerby. Externally it 
strikingly resembles the J/. Helmersiana, an oolitic fossil of 
Russia. 
2. M. moprotirormis, King. 
Pleurophorus modioliformis, King, Geol. Russ., p. 224 ? 
In Magnesian-limestone ; not uncommon. Humbleton, Tun- 
stall, Silksworth. 
This species is shorter and more tumid than the former, and 
very much resembles in general form some of the recent Modiole. 
It sometimes has a few radiating ribs on the posterior surface, 
but at other times it is almost smooth. 
20. AXINUS, Sowerby. 
1. A. TRUNCcATUS, King. 
Schizodus truncatus, 1845, King, Geol. Russ. i., p. 224; ii, 
p. 309. 
* Sowerby, Min. Conch., tab. 467. 
