178 
CRUSTACEA. 
Leucosia, Fah. 
Form of tlie shell varying, but generally ovoid or almost globular, 
and always very hard and stony; lateral antennae and eyes very small ; 
eyes approximated. The third joint of the external foot-jaws is 
smaller than the second, and without any apparent internal sinus ; 
these parts are contiguous inferiorly along the internal edge, and form 
an elongated triangle, the extremity of which is received into two 
upper cells of the buccal cavity. The tail, which is ample and subor- 
bicular in the females, usually consists of from four to five segments, 
but never seven. 
Doctor Leach * has separated this genus of Fabricius into several 
genera, which, however, we will consider as simple divisions. 
Those species which have a transversal shell, with the middle of its 
sides greatly prolonged or dilated, so as to resemble a cone or cylinder, 
forms his genus Ixa f . 
Those which have a rhomboidal shell with seven conical points, re- 
sembling spines on each side, compose that of Iphis. 
If the shell still has the rhomboidal figure, but merely presents 
angles or sinuses on the sides, it Ijecomes his genus Nursia. 
If these lateral edges are smooth, we have his Ebnlia. 
The Leucosiae witli an ovoid or nearly globular shell, and other- 
wise distinguished from several of the preceding by the claws being 
always longer than the body, and thicker than the other feet, and by 
the tarsi being sensibly striate, may be divided thus ; 
In some the front projects, or at least is not surpassed by tlie supe- 
rior extremity of tlie buccal cavity. The outer branch of the external 
foot-jaws is elongated, and almost linear. 
Here the claAvs are slender, the hands cylindrical, and the fingers 
long. 
Sometimes the shell is nearly globular, and either very spiny, as in 
the genus Arcania, or smooth as in Ilia. 
At others, the shell is suborbicular and depressed, as in the genus 
Persephona, or ovoid as in Myra. 
There the claws are thick, with ovoid hands and short fingers. 
I’liey constitute the true Leucosice of that naturalist. 
In the others, the superior extremity of the buccal caA^ity outreaches 
the front. The outer branch of the external foot-jaAvs is short, and 
arcuated ; the shell rounded and depressed. This last diA’^ision com- 
prises his genus Phylira. 
Other considerations, founded on the proportions of the feet and the 
form of the external foot-jaAvs, strengthen these characters. 
The Leucosie noyau; //fa nne/en,?, Leach ; Cancer nucleus^ 
Lin., Herbst., XI, 14, is common in the Mediterranean; its shell 
is globular, granulated on the sides and posteriorly ; the front is 
* Leach Zool. Misc. Ill; Desmar., Consid. 
t Leucosia cijHmlrus, Fabr., Herbst., II, 29 — 31 , 
