3G6 
CLASS CRUSTACEA. 
presence of a testa, in the form of an oval or semi-lunar 
buckler; by the number of visible feet, which is always 
twelve (or fourteen, if, with Dr. Leach, those may be considered 
as such, which I think to be the two inferior antennae) ; by 
the form and size of those of the last ten pair, which are 
sometimes multifid, pinnate, or terminated in a fin, and very 
well adapted at all periods, and in the adult state, for 
swimming ; sometimes in the form of leaflets, or broad and 
membranaceous. The sides of the thorax never present ex- 
pansions in the form of wings, inclining backwards, and they 
enclose the body posteriorly. 
In some the body, presenting several segments above, is 
elongated, and narrowed posteriorly, to terminate in the manner 
of a tail with two filaments, or two other projecting append- 
ages at the end. This extremity is not covered by a division of 
the superior teguments, in the form of a large rounded shell 
deeply notched at the posterior edge ; the testa occupies at 
least one half of the length of the body ; this subdivision 
will comprehend two genera of Muller. 
The first, that of 
Argulus, Midler, 
Was at first designated by us under the name of Ozole, and 
described in an incomplete manner. The younger Jurine has 
since observed the species which serves as its type, with the 
most scrupulous attention, has traced it in all its ages, and 
given us a monograph which leaves nothing to desire 
upon the subject. He has restored to this genus the name 
which Muller had originally imposed upon it. 
The arguli have an oval buckler, emarginated posteriorly, 
covering the body, with the exception of the posterior ex- 
tremity of the abdomen, supporting on a middle triangular 
space, distinguished under the name of hood, ( chaperon J two 
eyes, four very small antennae, almost cylindrical, placed in 
front, the superior of which, shorter and of three articulations, 
