174 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 21 



Atlantic analogue: Tyche emarginata White. 



Diagnosis: Rostral horns slender, exceeding preorbital. Length and 

 width of posterior medallion subequal. External maxilliped slender, 

 ischium grooved throughout length, basal projection of exognath recurv- 

 ing upon it, anteroexternal angle of merus not produced. First free 

 antennal segment cylindrical. Male first pleopod with a rounded protec- 

 tive terminal flap and a row of papillae. 



Description: Carapace very wide in front; gastric region swollen, 

 cardiac and branchial regions depressed. Lateral borders straight and 

 nearly parallel at hepatic regions, rounded at branchial regions. Pos- 

 terior border lamellate. Front with two small, flat horns curved toward 

 each other, extremities bispinose. The superior orbital border extending 

 in the form of a plate over the eye and projecting nearly forward in a 

 strong horn. A deep fissure separating orbit from hepatic margin. Cheli- 

 peds of male slender, smooth, shorter than the next pair of legs ; fingers 

 touching only at their extremity. Ambulatory legs very slender and cylin- 

 drical. (Rathbun, modified, after A. Milne Edwards) 



The foregoing description is no longer adequate in view of the 

 necessity for distinguishing the true Tyche lamellifrons of the Pacific 

 American mainland from the species newly described from adjacent 

 insular waters. The rostral horns are slender, overreaching the second 

 free antennal segment, and exceeding by a comfortable margin the pre- 

 orbital horns. The excrescence of the inner margin of each rostral horn 

 is comblike and the two together form an effective mechanism for the 

 anchoring of small bits of Ulva. The preorbital horns have the tips 

 broadened and the outer as well as the inner margins arcuate. The poste- 

 rior margin of the carapace is upturned as well as lamellate. The basal 

 antennal article may bear a small lobe at the anteroexternal angle, but 

 does not bear a spine. The groove of the ischium of the outer maxilliped 

 extends the entire length of the article, and the anteroexternal angle of 

 the merus is squared or rounded off, not extended in a thin blade. The 

 cheliped of the male is little more robust, that of the female no more so, 

 than the walking legs, which are exceedingly long, slender, and hairy. 

 Lastly, the male first pleopod is provided with a flap which is rounded, 

 rather than squared, on its inferior margin, and which scarcely conceals 

 the orifice. 



Material examined: 23 specimens from 18 stations. (See Table 30) 

 From Agua Verde Bay, Gulf of California, Mexico, to La Libertad, 

 Ecuador. 



