616 THE VOYAGE OF H.M.S. CHALLENGER. 



Chorismus, 1 n. gen. 



Carapace less than one-third of the length of the animal, posteriorly rounded, 

 anteriorly compressed, and produced to a long rostrum that is dilated on the under 

 surface anteriorly, and posteriorly excavated to receive the ophthalmopoda. Armed with 

 teeth on the dorsal crest and on the rostrum, above and below. No tooth on the orbital 

 region. Orbit defined by a tooth at the outer canthus. Frontal margin having an 

 antennal tooth, and one at the infero-lateral angle of the carapace. 



Pleon smooth, third somite arcuate, sixth somite twice the length of the fifth. 



Telson long, narrow and tapering. 



Ophthalmopoda pyriform, ophthalmus orbicular, ocellus imperfectly defined. 



First pair of antennae biramose, first joint furnished with a long, sharp stylocerite. 



Second pair of antennae having a long, slender flagellum, and a long scaphocerite that 

 has a rounded foliaceous extremity, and is armed with an outer distal tooth. 



Mandible furnished with a small and sharp pointed psalistoma, and a biarticulate 

 synaphipod placed at the base of the molar process, where it meets the apophysis. 



First pair of gnathopoda furnished with a long basecphysis. 



Second pair of gnathopoda without a basecphysis. 



First pair of pereiopoda robust and chelate. Second pair long, slender, and minutely 

 chelate, having the carpos long and multiarticulate. Posterior three pairs of pereiopoda 

 subequal, robust, and terminating in a sharp pointed unguiculate dactylos, which is armed 

 with stout spines on the posterior maigin. 



Branchiae consisting of seven pairs ; five pairs of pleurobranchise of nearly equal 

 prominence, one pair of small arthrobranchiee, and one of podobranchiae. The podo- 

 branchise are attached to the mastigobranchial plate of the first pair of gnathopoda. 



In external appearance this genus corresponds with Merhippolyte, but it is dis- 

 tinguishable from it by the character and number of the branchiae, in which respect 

 it more nearly resembles Spirontocaris, as may be seen in the following table : — 



Pleurobrancliiie, . . . 1 1 1 1 1 



Arthrobranchia?, . . ... 1 



Podobranchiae, . . . 1 



Mastigobranohiaj, . . . 1 r r r 



h i k 1 m n o 



It differs from Hippolyte in the absence of the supraorbital tooth, in having no 

 ecphysis to the second pair of gnathopoda, in having three instead of four small 

 mastigobranchias following the first pair of gnathopoda, and in having the carpos of the 

 second pair of pereiopoda multiarticulate, instead of triarticulate. 



Geographical Distribution. — There is only one species in the Challenger collection, 

 and that is abundant in the South Indian Ocean. 



1 xfitpwpo;, separation. 



