OF THE COUNTIES OF NORTHUMBERLAND AND DURHAN. 255 
which fills some of the cavities between the harder parts of the 
rock at Tunstall. In this state it can be easily cleaned from the 
surrounding matter, and shews the structure of the shell almost 
as conveniently as if it were recent. 
In general form it is round and tumid. The beak of the 
lower valve is slightly curved upwards, and the area is not well 
defined. The plaits on the surface and the extremities of the 
hinge-line are slightly rounded. These characters will readily 
distinguish it from the preceding species. The specimens from 
Dalton are much rounder than those from the other localities. 
A small shell, apparently belonging to this genus, is found in 
the lowest bed of limestone at Whitley and Pallion, but the spe- 
cimens we have seen were so imperfect that the species could not 
be ascertained. 
Famity. ORtTHIDs. 
32. ORTHIS, Dalman. 
1. QO. prLaRGonaTa, Schloth. 
Terebratulites pelargonatus, Schl., Mun. Ak. vi.,pl. 8, f. 21, 24. 
In Magnesian-limestone; rather rare. Dalton-le-Dale, Hum- 
bleton, Silksworth, Tunstall. 
This pretty shell is very irregular in general form. It much 
resembles, when seen from above, some of the recent Thecidea. 
It belongs, however, to an unattached genus, of which it is the 
only representative in the Permian system of England, and ap- 
parently one of the last species of the genus which existed. 
The lower valve is comparatively deep, and the beak is curved 
either to one side or the other. Between the beak and the hinge- 
margin is a large flattened area and a slightly convex distinct, 
closed deltidium. The upper valve is slightly convex, rather 
hollowed in the middle. The outer surface is covered with 
distinct radiating ribs, which are slightly raised, and bifur- 
cated near the margin. The lines of growth are distant till the 
shell attains nearly its full size, and then they are very close. 
The hinge-line of the lower valve is straight, and furnished with 
two small condyles, one on each side of the deltidium. In the 
other valve two callosites, one on each side of the umbonal cavity, 
