PART 1 GARTH : PACIFIC OXYRHYNCHA 377 



Atlantic analogue: None. A Galapagos Islands endemic species. 



Diagnosis: Oblique branchial sulci faintly indicated, especially in 

 large specimens. Three anterolateral marginal lobes, with the last, situ- 

 ated at the lateral angle, narrowest. Entire inner edge of cheliped to 

 middle of manus laminate. Male first pleopod fringed with hair, a 

 sheath extending beyond terminal opening. 



Description: Carapace widened, much depressed: front composed of 

 two very short frontal horns, obtuse, very close together. Preorbital 

 angles tuberculiform. External angle of the basal article of the antenna 

 prolonged into an obtuse point surpassing the preorbital angle. Lobula- 

 tions of the carapace less strong than in [related speciesl. Four tubercles 

 arranged transversely occupying the gastric region. Another line of larger 

 tubercles following the posterior margin above. Anterolateral margins 

 provided with three rounded expansions : the first, or hepatic, smaller 

 than the two following; a small spine placed behind the last. Develop- 

 ment of lateral expansions varying a great deal ; more considerable in 

 young females than in adult males. External maxillipeds differing a great 

 deal from those of other Mithrax. Merus broader than long, a little 

 notched on the inside for the insertion of the palp, palp with articles 

 broad and flattened. Posterior margin very concave on the outside [in- 

 side, according to Milne Edwards's figure] for the insertion of the sec- 

 ond article. External branch, or exognath, little widened. 



Chelae provided above with a crest occupying the posterior portion 

 of their margin. Another crest bordering the forearm on the inside. Arm 

 bearing several tubercles, its anterior margin advanced in a lamellate and 

 rounded prolongation, very broad in adult males, only spiniform in 

 females. Ambulatory legs spiniform, short and heavy. (A. Milne Ed- 

 wards, modified) 



Material examined: 850 specimens from 57 Galapagos stations. (See 

 Garth, 1946, pp. 392-395) 



Measurements: Largest specimen, male: length 30.1 mm, width in- 

 cluding lateral lobe 38.1 mm, without lobe 33.6 mm, rostrum 1.4 mm, 

 width 4.0 mm, cheliped 57 mm, chela 31 mm, dactyl 18.2 mm, height 

 of palm 13.4 mm, ambulatory legs 40, 35.5, 31, and 26.5 mm, respec- 

 tively. Female: length 23.5 mm, width including lateral lobes 28.1 mm, 

 without lobes 24.4 mm. 



Color in life: Uniform purplish brown. Hairs on legs and about 

 antennae yellow, in sunlight yellow green. (Garth) 



Habitat: Under stones at low water. (Bell) Shore pool. (Finnegan) 

 In coral. (Garth) 



