192 The American Naturalist. [March, 
solved the calcitic or lime parts of these animals, since they 
were not protected against this depletion by the presence of a 
calcareous matrix, unless, as with Obolella polita, at Trempeleau, 
Wisconsin, the great numbers of the fossil itself acts as a pro- 
tection against solution and attrition. The influence of aslight 
admixture of a plastic ingredient in improving the casts and 
impressions of fossils is seen in the more argillaceous layers of 
the Cambrian beds, as at Mazomania, Wisconsin, where Aglas- 
pis barrandii and Dicellocephalus minnesotensis are retained with 
a firmer and more legible outline. In many layers the Obolellas 
are broken, and appear as shelly fragments scattered over the 
rock. The Obolella is also a compact and small object, pro- 
tected to some extent by the convexity of its valves; but where 
it is preserved in perfection we may reasonably conclude, like the 
little Gemma gemma of our coasts, it had buried itself in the sand, 
and was not expelled from its safe position by waves or denu- 
dation. The regular close distribution of these fossils over the 
slabs of Potsdam sandstone show that they suffered little or no 
displacement. On the other hand, the fragments of Ptycho- 
paria (trilobite) in the same rocks at Trempeleau, Kickapoo, 
prove from their heterogeneous, confused interference of parts, 
some drifting, rubbing and dislocation. The Lingulepis pinni- 
formis, a delicate shell, and with valves held together by liga- 
ments alone, occurs in great numbers in the sandy Potsdam 
beds of St. Croix, and in a very good state of preservation; in 
some instances, the concentric lines of growth, the polished and 
corneous surface being retained. But, for the most part, the 
individual shells have suffered from friction and breaking. 
Lingulella stoneana in the red beds of Mazomania are flattened 
and disfigured. But in the green slates of Braintree, Mass., 
Paradoxides harlanii, a trilobite, is well preserved, except where 
distortion from compression and lateral motion has strained 
and deformed the parts. Similarly another crustacean, Olen- 
ellus, in the less dense slates of Georgia, Vt., also Lower Cam- 
brian, is found quite well preserved, but suffering mutilation 
and almost obliteration at times, from the metamorphic pres- 
sure by which these aluminous muds were transformed into 
fissile shales. In the annelid, green paper shales of Waterville, 
