OF THE COUNTIES OF NORTHUMBERLAND AND DURHAM. 25] 
compressed shell is oval and the surface is distinctly concentri- 
cally striated ; but all the specimens we have seen were too im- 
perfect to be determined satisfactorily. 
Famity. OrsicuLipa, Gray. 
28. ORBICULA, Cuvier. 
Two or three upper valves of an Orbicula were obtained from 
the Magnesian-limestone of Tunstall Hill, but they are so badly 
preserved that the specific characters cannot be correctly ascer- 
tained. 
Famity. TEREBRATULIDA, Gray. 
29. TEREBRATULA, Lhwydd. 
1. T. enoneata, Schloth. 
Terebratulites elongatus, Schloth., Petrif., pl. 20, f. 2. 
Terebratula elongata, Geol. Russ. ii., p. 66, pl. ix., f. 9 
a, b, c, d. 
In Magnesian-limestone; common. Humbleton, Tunstall, 
Silksworth, Dalton-le-Dale. 
This most common and characteristic Terebratula of the 
Magnesian-limestone is very variable in appearance. Some of 
the individuals are very convex, elongated, and truncated in 
front ; others are as correspondingly flattened, widened, and 
rounded. Generally, the perforated valve is slightly sinuated, 
and often has a distinct groove running down the middle. The 
other valve is slightly convex, with a slight indication of a sinus 
in themiddle. This shell is generally described as being smooth, 
but under the lens the surface is very distinctly shagreened. 
The imperforate valve is furnished with a simple loop-like 
process for the support of part of the animal, slightly re- 
sembling that of the recent 7. Australis. This character is 
sufficient to distinguish it from the 7. hastata, Sow., of the Moun- 
tain limestone, which has, according to Mr. M‘Coy, an internal 
support very distinct, and which slightly resembles that of the 
following species. Specimens of this shell have occurred nearly 
two inches in length, but generally they are not more than one- 
fourth of that size. 
VoL. I. ul 
