38 "GEOLOGICAL MEMOIRS. 
convex, with a few indistinet ribs: sides flat, their margins subsig- 
moid. Post-abdomen in rolling applied to the underside of the di 
domen, and then both to the underside of the head. 
= 
Yo me 
As to the mineral condition of the specimens, the buckler always 
presented (and more than 2000 specimens have been examined) a 
quantity of sulphuret of iron covered by a layer of sulphate of lime ; 
of these minerals the spines showed always a larger quantity than the 
fringe, and this again than the remainder of the head ; on the abdo- 
men and post-abdomen, when extended, no trace of these could be 
found, but im one case, when the body was rolled up and so applied 
to the head, a small quantity appeared. 
The state of preservation in which the parts of the body were found 
indicated also the greater or less proportion of lime im them. The 
spines were constant in shape and direction, or when altered, a decided 
fracture was visible. The fringe appeared to have been capable of 
considerable flexure, but when the two sides of the body had been 
compressed together a fracture with a clean edge took place towards. 
the outer border, but irregularly torn near the central parts of the 
head, which were apparently capable of bending im all directions. 
The abdomen and post-abdomen were generally deformed, the former 
most so, probably on account of a thinner texture. 
Therefore, the indications presented by the presence of the mine- 
rals which are substituted for calcareous matter, and by the degree 
of flexibility, tend the same way, and confirm each other ; the strong 
probability being, that the head was more calcareous than the rest of 
the body, the largest quantity of lime being accumulated in the ap- 
pendages, especially the strong spies; nor is the exception noted 
above, of arolled-up specimen showing traces of the pyrites, a real ob- 
jection, since the attraction must have been continued a short distance 
from the caleiferous portions. ) 
Then follows a table comparing the mineral condition of the crust 
in the following genera (at Rennes) :— 
Calymene, 64. «27-4 much pyrites, in slate. 
Proetas, noise detikve calcareous, in muddy limestones. 
PRAcopsy: a - sec little pyrites, in slate. 
Crypheeus ys --c, calcareous, in muddy limestones. 
Polyeres, n. g..... little pyrites, in slate. 
Prionocheilus, n. Lg. no pyrites, even while it is present near it. 
Cheraras® 322 "-. no pyrites. 
Illeenugy 4... ..2: no pyrites. 
Nileus-.........: no pyrites: 
Opveia, . rue. no pyrites. 
Trunuclenss =) 22 a pyrites in buckler only. 
In the table these genera are-also compared, with respect to the 
