Zoology.} NATURAL HISTORY OF VICTORIA. [Crmtacea. 



Plate 199. 



IBACUS PERONII (Leach). 



Peron's Ibacus Crab. 



[Genus IBACUS (Lbach). (Sub-kingdom Articulata. Class Crustacea. Order Decapoda. 

 Section ilacrura. Family Scyllaridae.) 



Gen. Char. — Carapace transverse, broader than long, depressed, with a very deep fissure on 

 each side. Rostrum bilobate. Eyes inserted on anterior edge, far removed from the external 

 angles ; peduncle thick, recurved, acuminated towards extremity. External antenna; not far 

 anart. squamiform, four-jointed ; internal antennas three-jointed, terminated by two many- jointed 

 setae. Branchife twenty-one. E.xternal pedipalps with two first joints externally crested, second 

 joint with many fissures internally.] 



Description. — Broad, ovate, male more convex in front, from greater width of 

 .second antennae, than female; greatest width, at otiter anterior angle of carapace, 

 equal to length from inner angle of orbit to base of telson or last abdominal joint ; 

 sides tapering from anterior lateral angles of carapace to penultimate abdominal seg- 

 ment in nearl}' straight or very slightly convex lines ; space between orbits one-fifth 

 the width of anterior edge of carapace; outer angle a large flat spine curving out- 

 wards and slightly forwards from bottom of deep lateral fissure, posterior edge, and 

 to less extent the anterior edge, more abruptly arched forwards and a little outwards; 

 lateral fissure one-fourth the width of carapace at second lateral spine, narrow, 

 parallel-sided, less transverse, or inclined obliquely more forward than anterior 

 margin of carapace, both edges imperfectly crenulated with a row of minute tubercles, 

 lower edge fringed with long hairs; six angular, gradually decreasing teeth or 

 serratures on each side behind fissure, sometimes a smooth, most posterior, rudimen- 

 tary one*; anterior edge of carapace irregularly serrated by numerous, small, unequal 

 tubercles, arched forwards at outer angles; upper surface of carapace much flattened 

 at the sides, thicker in the middle third, bounded by two lateral smooth keels 

 arising in line with middle of orbits and diverging backwards, so that space between 

 hinder ends is rather less than half the width of the carapace, while between the 

 anterior ends it is less than one-third the width at same level ; middle keel divided 

 into four long tubercles; surface nearly smooth, finely granular at margins; granu- 

 lation coarser behind posterior sulcus and within lateral keels. Outer antennre, with 

 first joint .small, scarcely extending beyond rostrum, second joint very wide, posterior 

 edge smooth, outer edge with five teeth, anterior largest; anterior edge irregularly 

 serrated with small, irregularly unequal, spinose tubercles, inner ones largest ; third 

 or anterior joint with convex anterior edge with three large angular teeth; anterior 

 edge of sinus, of carapace, and anterior outer and inner edges of outer antennae 

 fringed with hair. Inner antennce with three, slender, long joints ; flagella many- 

 jointed, the outer fringed with a large tuft of hair. Four anterior abdominal .seg- 

 ments (after the first) each with a longitudinal tubercle in middle continuing the 

 median keel of carapace ; sides serrated and fringed with hair ; all granulated ; fifth 

 segment without median tubercle, but with two transverse rows of small granular 

 spines on hind edge of terminal or coxal pieces, the outer forming an obscure point ; 

 sixth joint with granulated hind edge. Telson and lateral flaps of tail. soft and 



* Although Leach says five, his figure shows six, as in our specimens, 

 V»t. IL-DlCABB XX.— 3/. [ 369 ] 



