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THE VOYAGE OF H.M.S. CHALLENGER. 
the margins subentire and very little prominent. Front narrow, with the anterior 
margin nearly straight. Epistoma very short and transverse. Buccal cavity wide and 
transverse, and anteriorly arcuated, the endostomian ridges obsolete or nearly so. Post- 
abdomen narrow and distinctly seven-jointed, its base does not cover the whole width of 
the sternum between the coxge of the fifth ambulatory legs. Eye-peduncles very short. 
Antennules transversely plicated wdthin the small fossettes, which are little broader than 
long. Antennae very small and placed within the anterior hiatus of the orbit ; basal 
antennal joint slender and short, barely attaining the infero -lateral angle of the front. 
Exterior maxillipedes oblique, with the ischium-joints of the endognath short or rudi- 
mentary, the merus very large, usually curved, and widening more or less to the distal 
extremity, which is rounded or subtruncated, the small dactyl articulated on the interior 
margin of the penultimate joint. Chelipedes in the male small, with the joints smooth, 
merus trigonous, carpus without a spine on its anterior margin, palm slightly compressed 
and rounded above and below, fingers acute. Ambulatory legs of moderate length and 
very slender, with the joints smooth; dactyli slightly arcuated and usually shorter than 
the penultimate joints. 
The species are all of small size, and (as is we]l known), inhabit the shells of Mytilus, 
Pinna, and other bivalves, and are found probably in all the temperate and tropical 
regions of the globe. 
The following species are either not referred to or have been added since the publica- 
tion of Milne Edwards’s Memoir in 1853. As the discrimination of the species is difficult, 
and a revision of the genus is much needed, it is possible that some are not really 
specifically distinct. 
Pinnotheres maculatum, Say. 
Pinnotheres pholadis, de Haan. Japan. 
Pinnotheres obscurus, Stimpson. Hong-Kong. 
Pinnotheres boninensis, Stimpson. Bonin Island. 
Pinnotheres parvulus, Stimpson. China Seas. 
Pinnotheres margarita, Smith, Bay of Panama, Pearl Islands, and Lower 
California. 
Pinnotheres lithodomi, Smith. Pearl Islands and Lower California. 
Pinnotheres ascidiicola, Hesse. Coast of France. 
Pinnotheres pectunculi, Hesse. France. 
Pinnotheres angelicus, Lockington. Gulf of California. 
Pinnotheres flaws, Nauck. Philippines. 
Pinnotheres byssomise, Say. 
Pinnotheres depressum, Say. 
Pinnotheres monodactylum, Say. 
Southern Coasts of 
