_, CRUSTACEA. 



64 



important distinction. In many other Crustacea, like the. Cancroidea 

 the basal joint is situated inward of the eyes, and more posterior, and 

 rarely projects beyond them; besides, instead of being soldered con- 

 tinuoLisly with the shell outside, it is separated by a distinct suture, 

 though still immoveable. In other species, still, the antennae have 

 free motion from their very base. These are both marks of a lower 

 grade than that exhibited by the concentrated and consolidated con- 

 dition in the Maia type. 



The outer antennse are usually quite small and naked, or nearly so, 

 and extend forward and outward. But in one group (the Corys- 

 toidea), they are generally long and hairy or ciliate, and extend 

 inward and forward. 



In structure, form, and organs of locomotion, the differences are 

 mostly of small value, characterizing genera rather than higher groups. 

 Yet form is important, when sustained by other characters. Among 

 the Leucosoidea there is a strange diversity of shape ; the broad con- 

 vex Hepatus, the still broader Calappa with thin extended sides, the 

 globular Ilia, and narrow-snouted Leucosia, Ixa with its sides length- 

 ened into cylinders, and the thorny Iphis, make a fantastic group; yet 

 all are of one tribe. But among the other Brachyura there is much 

 less variation. The oblong Maia, narrowing anteriorly, is widely 

 diverse from the broad Cancer, with its arcuate front margin, and as 

 diverse from the square Grapsus; and these several forms are charac- 

 teristic of as many groups, though liable to variations of considerable 

 amount. The narrow head of the Maia throws the bases of the eyes 

 almost in contact, and places these organs over the base of the outer 

 antennae; at the same time, while the front is elongated, the epistome 

 is long for its breadth, and a narrow, oblong space is left for the inner 

 antenna), which are therefore longitudinal. The broader form of the 

 Cancer allows the eyes to be distant, the base of the outer antennas to 

 be interior to them instead of directly beneath, and as the front also 

 is not produced, the epistome is very short, and the inner an ten me 

 are usually transverse. Yet even with the broad form, the bases of 

 the eyes may be nearly in contact, as in Ocypoda; and since in these 

 species the front is not lengthened into a beak, it follows that the an- 

 tennary space directly beneath or adjoining the narrow frontal piece is 

 exceedingly reduced in size, and the antenna) are minute. This is 

 also true of the Oxystomes, in which the front is very narrow. 



Milne Edwards, with his usual acumen, has also pointed out pecu- 



