292 The American Geologist. November, looi. 
ing grounds, teeming with life. The favorable conditions are 
shown bv the large size of the trilobites, Calymene and Cerau- 
rus. The head shields of the latter often indicate individuals 
five or six inches long. In these facts we find an explanation 
for the occurrence in these lenticular thickenings of communi- 
ties of brachiopods, for the aggregates of cephalopod shells, 
for the great numbers of trilobite moultings, and for the 
abundance of many otherwise rare forms. 
The manner of occurrence of the trilobites is striking. 
Their exuvice are commonly found in groups or pockets, some- 
times of one species, but more often of two or three. They may 
appear to be entirely absent from some masses, or to occur in 
two or three parts, or at distinct levels in the same mass. The 
head and tail shields of Illacnus are almost invariably inverted 
and generally packed together, one inside the other, like broken 
egg-shells. These cachements may represent the action of 
light currents in shifting the tests into depressions in the irreg- 
ular surface. But the position of some of these moultings sug- 
gests that they were left in crannies and silt-lined depressions 
which were inhabited by trilobites, attracted to these growths 
by the food supply. Outside of the lenses trilobites are rare. 
Cephalopods are not unusual in the surrounding limestone 
but in the lenses they occur grouped like the trilobites. They 
are commonest in those masses in which the trilobites are 
found, and the two forms are frequently commingled. All 
forms, especially brachiopods, lived in communities upon these 
sites; IVhitficldcUa nitida often occurring by the hundreds. 
It thus appears that the comparative richness of these sites 
is due (i) to the conditions attractive to all forms of life, grow- 
ing out of the advantages the areas afforded to attached forms ; 
(2) to the anchorage and shelter furnished after death by 
the bryozoan framework; (3) to the groupings of certain,, 
sometimes conspicuous forms, an outcome of their colonial 
habits ; and (4) to the partial exclusion of silt, the fine material 
found in these sites being partially formed by decay and gentle 
trituration of delicate organisms. 
This wealth of material and its fine preservation is valuable 
to the collector, for in these lenses, where well weathered, more 
material illustrative of the fauna of this clear limestone-form 
ing sea may be found in a much shorter time, than can be ob- 
tained in the including beds. 
