178 CRUSTACEA. 



Leucosia, Fab. 



Form of the shell varying, but generally ovoid or almost globular, 

 and always very hard and stony; lateral antcnnte 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 genvis 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 rhombuidal shell with seven conical points, re- 

 sembling spines on each side, com^wse that of Iphis-, 



If the shell still has the rhomboidal figure, but merely presents 

 angles or sinuses on the sides, it becomes his genus Nwiia. 



If these lateral edges arc smooth, we have his Ebalia. 



The Leucosiae with 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 the supe- 

 rior extremity of the buccal cavity. The outer branch of the external 

 foot-jaws is elongated, and almost linear. 



Here the claws are slender, the hands cylindrical, and the fingers 

 long. 



Sometimes the shell is nearly globular, and either very spiny, as in 

 the genus Arcititia, 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. 



Tliey constitute the true Leucosicr of that naturalist. 



In the others, the superior extremity of the buccal cavity outreaches 

 the front. The outer branch of the external foot-jaws is short, and 

 arcuated ; the shell rounded and depressed. This last division com- 

 prises his genus Phylira. 



Other considerations, founded on the proportions of the feet and the 

 form of the external foot-jaws, strengthen these characters. 



The Leucosie noyau ; Ilia nucleus, 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. 



■f Leucosia cylimlrus, Fabr,, Herbst., II, 29 — 31. 



