256 GATALOGUE OF THE FOSSILS OF THE PERMIAN SYSTEM 
diverge from the middle of the hinge-line, and are produced into 
two elongated processes which fit into the deltidium of the lower 
valve. A socket on each side of these processes receives the arti- 
culating condyles of the other valve. This species is said to oc- 
cur in the Zechstein of Germany, and a closely allied species, 
the 0. Wangenheimei, is found in the Russian Permian rocks. 
Famity. Propvctipa, Gray. 
33. PRODUCTUS, Sowerby. 
1. P. Horripus, Sow. 
Productus horridus, Sow., Min. Chon., tab. 319, f, 1. 
Productus caluus, Sow., Min. Chon., tab. 560, f. 2—6. 
Gryphites aculeatus, Schl., Min. Taschb. vii, pl. 4, f. 1, 2, 3. 
In the Magnesian-limestone; common. Whitley Quarries, 
Humbleton, Tunstall, Dalton-le-Dale, Garmonsway. 
P. horridus is very characteristic of the Magnesian-limestone, 
as it is the largest and oftentimes the most abundant species. 
The specimens from the lowest compact limestone have been de- 
scribed as a species, P. calvus, by Mr. Sowerby. They are 
sometimes larger than those which occur at Humbleton. As 
only part of the shell is generally preserved, the spines are not 
always visible. This appearance has led to the erroneous conclu- 
sion that this variety was spineless, and consequently distinct 
from P. horridus. 
In this species the hinge-line cf both valves is furnished with 
a row of large spines. The spines are hollow, and seem to have 
served as a protection for filaments or tentacular appendages to 
the mantle or some other part of the body. The spines on the 
body of the shell were closed at the base as often as it became 
necessary for the animal to enlargeit. After the shell was suf- 
ciently enlarged, new spines were formed in lieu of the old ones, 
which were now no longer serviceable. 
2. P. LATIROSTRATUS, 2, S. 
Shell grypheoid or concavo-convex ; lower valve con- 
vex bilobed, or with a slight furrow or sinus in the centre ; 
upper valve slightly concave, or nearly flat; beak of lower 
or convex valve, large, and very much flattened ; hinge line 
