J. D. Dana on the Classification of Crustacea. 11 
of the 'Tetradecapods. "There is thus another analogy between 
this group and the Anomoura. 
. The Trilobita probably belong with this second type, rather 
than the Entomostracan. Yet they show an abberrant character 
in two important points. First, the segments of the body are 
multiplied much beyond the normal number, as in the Phyllopoda 
among the Entomostraca; and Agassiz has remarked upon this 
as evidence of that larval analogy which charaeterizes in many 
cases the earlier forms of animal life. In the second place, the 
size of the body far transcends the ordinary lsopodan limit. 
This might be considered a mark of superiority; but it is more 
probably the reverse. It is an enlargement beyond the normal 
and most effective size, due to the same principle of vegetative 
growth, which accords with the inordinate multüplication of seg- 
ments in the body. 
The third primary type (the Entomostracan) includes à much 
wider variety of structure than either of the preceding, and is 
less persistent in its characteristics. tis, however, more remote 
in habit from the Tetradecapods, than from the lowest Decapods, 
and is properly a distinct group. Unlike the Decapods and Te- 
iradecapods, there are normally but si» annuli devoted to the 
senses and mouth in the highest of the species, and but five in 
others, the mouth including a pair of mandibles, and either one 
or two pairs of maxille (or maxillipeds). 'lhis is an abrupt step 
below the 'l'etradecapods. We exclude from these mouth organs 
the prehensile legs, called maxillipeds by some authors, as they 
are not more entitled to the name than the prehensile legs in 
Tanais, and many other Tetradecapods. "There is an exception 
to the general principle in a few species. .À genus of Cyproids 
has three pairs of maxille; but this may be viewed as an exam- 
ple of the variations which the type admits of, rather than as an 
essential feature of it,—possibly a result of the process of obso- 
lescence which marks a low grade, as in the Myside, whose 
abdomen by losing its appendages, approximates in this respect 
to the Brachyural structure, though, in fact, far enough remote. 
The species of the Entomostracan type show their inferiority 
to either of the preceding in the absence of a series of abdominal 
appendages, and also in having the appendages of the eighth, 
ninth, tenth, and eleventh normal rings, when present, natatory 
in form. 
The range of size is very great,—and this is a mark of their 
low grade, for in this respect they approach the Radiata, whose 
limits of size are remarkably wide. Nearly all of the species, 
and those which, by their activity, show that they possess the 
typical structure in 1ts highest perfection, are minute, not avera- 
ging over a line in length, or perhaps more nearly three-fourths 
of a line. 
