40 The American Geologist. Juij , i897 
(2) The segmentation of the protaspis is very feeble in 
the earliest stages, and is evidently emphasized from the fact 
that the fossils are viewed as opaque objects and exhibit 
strongly any inequalities of surface features, while living 
nauplii are studied as translucent objects. Furthermore, any 
such difference cannot be real, since the nauplius shows its 
true segmented nature in its paired appendages. 
(3) The apparent absence of a median eye in the trilobite 
protaspis could be taken as of some value were it not that 
the fossils are not more than one millimetre in length, and 
even under the most favorable conditions could hardly be ex- 
pected to show such small features as ocelli. Moreover, the 
median eye may have been marginal or ventral and therefore 
would not be seen in the fossil, which only preserves the dor- 
sal crust. 
(4) Paired eyes are not present, or at least not visible in 
the protaspis stages of primitive trilobites. They may through 
acceleration appear in the protaspis stages of later genera, as 
they do in the nauplius embryos of certain modern decapods. 
I do not believe that the nauplius has any great phyloge- 
netic significance, and have considered it "as a derived larva 
modified by adaptation" (?. c, p. 190), and as a "modified 
crustacean larva" (ibid., p. 191). 
It does not seem necessary to correlate the post-oral second 
pair of trilobite appendages with the mandibles of higher 
Crustacea. The second pair in the nauplius is also post-oral 
and manducatory, though they later develop into the antennae 
and are pre-oral. 
As to the eephalon of a primitive crustacean, I have mere- 
ly accepted the conclusion approved b}^ Claus, as stated by 
Lang, in his reconstruction of the original crustacean, which 
is as follows: "The head segment was fused with the four 
subsequent trunk segments to form a cephalic region" (Com- 
parative Anatomy, p. 406). 
Similarly in regard to the interpretation of the biramous 
appendages, I have adopted the statements and conclusions 
of a large number of zoologists who consider the most primi- 
tive appendages as branched or consisting of a dorsal and a 
ventral member, and I have followed them in thus interpre- 
ting the trilobite appendages, which are clearly of this nature. 
Yale University Museum, I 
Neio Haven, Conn., April 14, 1897. \ 
