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



DEFINITION OF THE FAMILY. 



Nebaliid.e. 



Anterior part of body covered by a large compressed bivalvular carapace, connected 

 Tvith the body only along the cephalic part, its valves admitting of being moved by a 

 distinct adductor muscle and extending down the sides so as to enclose between them 

 all the oral parts as also the greater part of the other Limbs. A tongue-shaped rostral 

 plate present in front, movably articulated to the carapace. Trunk covered over by the 

 carapace and composed of eight subequal segments. Posterior part of body tapering 

 backwards and consisting of two more or less distinctly defined subdivisions, pleon and 

 tail, each composed of four segments. Eyes j^edunculated and mobile, but without 

 fiicetted cornea. Both pairs of antemiEe strongly developed, subpediform, with the 

 jaeduncle geniculate; anterior pair, or antennulee, with the peduncle four-jointed and 

 bearing a setose lamella at the end, besides the flagellum; posterior pair with a single 

 multiarticulate flagellum. Mandibles comparatively small, with the cutting part rudi- 

 mentary, the molar tubercle well develoj^ed, palp very large, triarticulate. Fii-st pair of 

 maxillae with two incurved masticatory lobes and a very elongate and slender reflexed 

 palp; second pair lamellar, with distinctly defined palp and exognath. Eight pairs of 

 subequal phyllopodous legs present on the trunk posterior to the oral parts; endopodite 

 more or less produced, with the inner edge and apex densely setiferous, but without 

 projecting lateral lobes. Four pairs of powerfully developed biramose pleopoda on the 

 succeeding part of the body, followed by two pairs of rudimentary caudal limbs. 

 Penultimate segment without limbs. No telson. Caudal rami simple, forming two 

 diverging plates edged with spinules and setse. Ova deposited within the lower part of 

 the carapace, and supported between the branchial legs. Development direct, without 

 metamorphosis. Internal organisation on the whole rather similar to that in the 

 Amphij)oda. 



Remarks. — As the fossil forms referred to the Phyllocarida are still very imperfectly 

 known, none of the limbs having as yet been found preserved, it is rather difficult to 

 point out the characters which should be regarded as exclusively distinguishing the 

 recent family Nebabidas. There is however at least one character, well seen in the 

 fossil forms, which seems to distinguish them very sharply from the Nebaliidce, viz., the 

 presence of a strongly developed telson, no trace of which is found in any of the recent 

 forms. 



As above stated the recent Phyllocarida were formerly only represented by a single 

 genus, Nehalia. The two new genera, added by the Challenger Expedition, chiefly 



