OF THE COUNTIES OF NORTHUMBERLAND AND DURHAM. 247 
terior end, and as the ligamental area is small they are rather 
close. 
It may be easily distinguished from the A. tumida, Sow., by 
its more elongated and less tumid appearance. The anterior is 
rather longer and rounder, and the posterior much longer and not 
so sharply truncated. The margins are more nearly parallel and 
the ligamental area is smaller. It is also distinguised from the 
A. antigua, Minst., of the German Zechstein, by its less ovate 
and ventricose form. At first we were inclined to consider it 
identical with the A. Lacordairiana, of M. de Koninck, but it 
differs from that species in many particulars. 
We dedicate this species to W. K. Loftus, Esq., of Newcastle, 
to whom the naturalists of this district are indebted for the dis- 
covery of the Chiton at Tunstall-hill. 
2. A. TumIDA, Sow. 
Arca tumida, Sow. Min. Conch., tab. 474, f. 3. 
In Magnesian-limestone ; common. Humbleton, Tunstall, 
Silksworth, Dalton. 
This characteristic species is very tumid towards the anterior 
portion of the shell. The beaks are very distant, in consequence 
of the great size of the ligamental area. The surface is covered 
with strong radiating ribs, which rarely bifurcate, but gradually 
widen towards the lower edge of the shell. These ribs are dis- 
tinctly granulated on the posterior slope only. The free margin 
is very much curved, and the curve for the byssus is very large. 
The individuals of this species are generally well preserved. 
Many of them were bored by some carnivorous animal before 
they were imbedded in the rock. 
3. A. Kinerana, d Verneuil. 
Arca Kingiana, VVern., Geol. Russ. 11., p. 313, pl. xix.,f.11. 
In Magnesian-limestone ; rather rare. Tunstall-hill. 
This species is not very common in our limestone. The speci- 
mens are rather small. Some of them have a few obsolete 
diverging ribs on the middle and posterior portions of the shell, 
but do not otherwise differ from the description given of the 
Russian specimens. 
