116 
Short spines. The surface of var. « is generally very 
flat in the middle ; but some specimens are concave, and 
lead to the larger variety, from which they differ only in 
size. The spines upon each side the beak are in a single 
row, and few. Jt is difficult to detect them except in 
casts. Var. 6 much resembles P. horrida (t. 319. f, i, 
Var. a (fig. 2.) was found some years back, and for- 
warded to us by Samuel Hailstone, Ksq., at Nosterfield 
near Tanfield, north of Ripon, Yorkshire, in Magnesian 
Limestone, where Prof. Sedgwick has since observed it. 
Var. 6 (figs. 3 and 4) are from a bed of compact gray 
Magnesian Limestone, immediately above a yellow 
Marl-slate, containing at least six species of fish, at East. 
Thickley, near West Aukland, Durham; while the casts 
represented at figs. 5 and 6, are from the higher bed of 
yellow Magnesian Limestone of Humbleton Hill and 
other quarries near Sunderland, towards Durham. 
In the series of figures given upon this plate, we have 
a complete illustration of the genus Producta. Figs. 1, 
2, 3, and 4, exhibit various views of the outer surfaces 
of the two valves, as they appear when separated from 
the stone; in which case the spines are commonly lost. 
Fig. 6. shows the cast of the same surfaces with the im- 
pressions of the spines near the hinge line. Fig. 5. ex- 
hibits a cast of the inside ; it discovers the impressions 
of the muscles and tendons that gave the valves motion, 
a short septum in the lesser valve, and the impressions 
of the short spines of its scabrous surface. 
