Natural History of the United States. 339 
Form, under rather wide limits of range, distinguishes the 
Orders; with narrower limits, the next grade of Orders; very 
distinctly, the third grade, the Maioids being called triangular 
Crabs, etc., and still more distinctly and usually without the aid 
ofordinal distinctions, Families, or the next range of subdivisions, 
We do not undertake at this time to inquire particularly into 
the subordinate groupings. 
e Brachyura, on what may be called their normal level, are 
geatly multiplied in species, and among either the Maioids, 
croids or Leucosoids, difference of rank is little apparent in 
© gteat majority of species. Here Families may be distin- 
gushed by form, though hardly by this exclusively. But there 
are genera in which the groups decline from the normal level, 
and the decline once begun, goes on with rapid increase from 
one genus to another. Among the Maioids, the decline is seen 
tnctly in the Parthenope group which has not the close com- 
pact head and head organs of the Maioids, but approximates in 
’ becoming a more and more rapid descent, so that grade is 
observed even among Families and also Genera, because the type 
8 here « drawn out long”; (8) the small number of species exist- 
| ie below the normal level along the declining grade. The num- 
‘of ordinal degrees of subdivisions in Crustacea—that is, sub- 
livisiong in me ade is distinctly marked,—is not necessarily 
a W for other groups, because, as before said, this class is few in 
i, _°S and is expanded over an immensely wide range of grade, 
( wiking contrast with the class of Insects. The same number 
f degrees of subdivisions might have existed without distinct 
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