92 The Anterican Geologist, February, 1895 
pygidiiuii luive been described.* There yet remain for inves- 
tigation the appendages of the head and additional details of 
other parts of the animal. These characters have not been 
easily obtained on account of the labor of removing the rock 
from such delicate structures. Moreover, but few specimens 
are in the proper position or condition to yield satisfactory 
results. The appendages of the head either suffered greater 
decomposition than those of the thorax, before mineralization^ 
or were so tenuous as to be easily obliterated, and are now 
seldom sutticientl}^ well preserved for study. Further, the 
number and compact arrangement of such complicated organs,, 
even when present, make it difficult to trace their precise 
form. A similar ditticulty would be experienced were one to 
endeavor to describe the appendages of Apvs by examining 
the ventral side without cutting awaj^ some of the limbs or at 
least unfolding or. bending them around. 
The features described in the present paper have been ob- 
tained by further work on the material secured for the Yale 
Museum, b}" professor Marsh. No detailed review of the 
ventral anatomy of Triarihrns will be given at this time, only 
such additional characters as have been observed since the 
publication of the last paper on this trilobite. The precise 
structure and relations of the organs here described must also 
be left subject to slight modifications required by researches 
which are still in progress. The writer has carefully prepared 
the specimens and made the drawings from camera lucida 
outlines. The appendages, however, are often so faintly pre- 
served or so obscure that in order to represent them in a pen- 
drawing it is necessary to emphasize their limits and their 
prominence, and this may sometimes lead to errors of inter- 
pretation. It seems almost unnecessary to state that errors 
are not due to any preconceived notions of trilobite anatomy^ 
*W. D. Matthew. — On Antonnce and other appendages of Triarthriis 
Beckii. N. Y. Academj- of Sciences, May, ]8S):i; American .loiirnal ol' 
Science, August, 181)3. 
C. 1). Walcott. — Note on some Appendages of the Trilol)ites. Proc. 
Biol. Soc, Washington, Marcli, 1894. 
C E. Beecher. — On the Thoracic Legs of Triartlinis. American .lour- 
nal of Science, December, ]89'5. 
On the mode of Occurrence, and the Structure and 
Development of Triarthrus Becki. AmkkicanGeoi.O(!Ist, January, 1894. 
The Appendages of the Pygidinm of Triarthrus. 
American Journal of Science, April, 1894. 
