Triarthrus Becki. — Beecher. 39 
nish interesting facts as to the mode of occurrence, and to the 
habits of the trilobite. 
In their present condition, the specimens contain very lit- 
tle calcite matter, and nearly the entire calcareous and 
chitinous portions of the animal are represented by a thin film 
of iron pyrite. To this kind of fossilization is doubtless due 
the preservation of delicate organs and structures, which oth- 
erwise would have been destroyed. For, as is well known, pyr- 
ite may replace such organic tissues as chitine or even soft 
dermal structures, the change occurring by the slow decom- 
position of these tissues in the presence of iron sulphate in 
solution, or from the action of hydrogen sulphide as a result 
of decomposition in a chalybeate water. 
From the mode of occurrence of the specimens, it is evident 
that some physical change of a rather sudden nature must be 
inferred to explain the facts. This is shown from the follow- 
ing considerations : ( 1 ) their restricted vertical distribution ; 
(2) nearly all specimens are complete and preserve their ap- 
pendages; (3) they are of all ages, from larval forms up to 
full-grown individuals; (4) the rock has a characteristic 
structure and composition; and (5) the adjacent strata con- 
tain a rather sparse fauna in which the trilobites are gener- 
ally fragmentary, or usually without appendages. 
It does not require a violent catastrophe to account for 
these peculiarities, and, as in the case of the recent destruc- 
tion of the tile fish off the eastern coast of the United States, 
it is possible that a temporary change in the direction of an 
ocean current, with the consequent variation of temperature, 
would be amply sufficient. Just what occurred in the present 
instance has not been determined. Throughout the trilobite- 
bearing rocks generally, young and larval forms are extremely 
rare, while, of full-grown examples, fragments are the rule and 
entire specimens the exception. Therefore, it is believed that 
the remains commonly found represent sheddings or moults, 
and not in each case the death of a separate individual. In 
the present material, however, the almost invariable perfection 
of the specimens precludes this view. Moreover, the append- 
ages are apparently in the position held in life, and not such 
as obtain in the cast-off shells of recent erustacea. 
Another feature noticed in the field is that the specimens 
