OF THE COUNTIES OF NORTHUMBERLAND AND DURHAM. 253 
occur with the former at Humbleton, and which seem to have 
been hitherto regarded as belonging to the same species. After 
examining an extensive series, however, of both these forms, we 
are obliged to conclude from the following constant characters 
that they are quite distinct. In the present species the form is 
more ovate, and the sinus is much wider and less elevated in the 
front. The plaits are always more numerous, and when they 
are not bifurcated they are more nearly parallel than in the pre- 
ceding species. It is.also of a much larger size, and occurs only 
in one locality. 
4. T. CORYMBOSA, 7. s. 
Shell nearly globular, about the size of pea; perforated 
valve sinuated in the middle; imperforate, very convex 
and high im the centre; surface, with two distinct plaits 
in the sinus, und one on each side, granulated. 
In Magnesian-limestone; not rare. Humbleton, Tunstall, 
Dalton-le-Dale. 
This small species, like the preceding, appears to have been 
confounded with the 7. Schlotheimet, with which it is generally 
found. It is, however, always more spherical, and has, uniformly, 
two plaits only in the sinus, and one on each side. The small 
size and rounded form distinguished it instantly from either of 
the former species. 
Famity. SPIRIFERID, Gray. 
30. ATRYPA, Dalman. 
1. A. PECTENIFERA, J. Sowerby. 
Atrypa pectenifera, Sow., Min. Conch., vii., pl. 616. 
Terebratula pectenifera, Geol. Russ. ii., pl. 8, f. 12 a, b. 
In Magnesian-limestone ; not rare. Humbleton Hill. 
This interesting shell, though very characteristic of the Mag- 
nesian-limestone, is yet very local in this district, occurring only 
in one locality. It is an orbicular lens-shaped shell, having the 
surface, when perfect, ornamented with lamelliform rows of close- 
set spines. The apex of the lower valve is furnished witha large 
perforation, and the internal processes are similar to those of 
Spirifer. 
