172 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL.21 



Genus TYCHE Bell 



Tyche Bell, 1835b, p. 172; 1836, p. 57. A. Milne Edwards, 1878, p. 



125. Rathbun, 1925, p. 507. 

 Platyrinchus Desbonne, 1867, p. 3; type: P. trituberculatus Desbonne, 

 1867 = Tyche emarginata White, by monotypy. 



Type: Tyche lamellifrons Bell, by monotypy. 



Description: Carapace oblong-oval, flattened, with lamellate exten- 

 sions in front and behind; front very wide, with four horns, the two 

 lateral forming anterior angles of orbit. Orbits covered by a lamellate 

 prolongation concealing the ocular peduncles nearly to their extremities ; 

 no orbital floor below, the ocular peduncles being entirely uncovered. 

 Antennae concealed beneath rostrum ; basal segment somewhat enlarged, 

 a small blunt point at its anteroexternal angle ; following articles cylindri- 

 cal. External maxillipeds exhibiting a very remarkable arrangement, not 

 shown in any other genus: external branch or exognath narrow and 

 having at its base a falciform prolongation bent around forward and 

 lodged in a groove of the ischium of the endognath. Merus much extended 

 backward and then inward, cutting deeply the anterior border of the 

 ischium. 



Chelipeds little enlarged. Ambulatory legs slender, with very hooked 

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



The description given above is emended as follows to include one of 

 the new species here proposed: the ischium of the endognath of the 

 external maxilliped may or may not be grooved, in which latter case the 

 recurving projection of the exognath finds no lodgment therein. 



The males of the genus are seven-segmented (Bell) ; the females 

 possess seven free abdominal segments also. The male first pleopod is 

 bent near the apex, which is sharply or bluntly pointed and lacks supple- 

 mentary processes, although the end may be split to form a flap which 

 protects the orifice. 



Range: Eastern Pacific from Agua Verde Bay, Gulf of California, 

 and Cape San Lucas, Lower California, Mexico, to Salango Island, 

 Ecuador ; Clarion and Galapagos Islands. Western Atlantic from Florida 

 Keys to Cape St. Roque [Sao Roque], Brazil. Shore to 30 fathoms. 



Key to the Pacific American Species of Tyche 



la. Rostral horns exceeding preorbital. Length and width of pos- 

 terior medallion subequal. Ischium of external maxilliped 

 grooved, basal projection of exognath recurving upon it. First 



