MALACOSTRACA. 1 13 
fied, but exopodite and endopodite are generally represented at least by rudiments. The 
fourth post-oral limb is a lobed plate, usually not distinctly segmented, and the fifth a uni- 
ramous pediform leg. The sixth, if present at all, is vestigial. 
Very little comparison can be made between the Ostracoda and Trilobita, other than 
in the ground-plan of the limbs, but the presence of biramous antenna: is a primi- 
tive characteristic. 
CIRRIPEDIA. 
Like the ostracod, the adult cirriped bears little external resemblance to the trilobite. 
The form of the nauplius is somewhat peculiar, but it has the typical three pairs of appen- 
dages, to which are added in the later metanauplius stages the maxilla; and six pairs of 
thoracic appendages. In the adult, the antennules, which serve for attachment of the larva, 
usually persist in a functionless condition, while the antenna? disappear. The mandibles, 
maxillula?, and maxilla? are simple and much modified to form mouth parts, and the six 
pairs of thoracic appendages are developed into long, multisegmented, biramous appendages 
bearing numerous seta? which serve for catching prey. Paired eyes are present in later 
metanauplius stages, but lost early in the development. The relationship to the trilobite evi- 
dently is not close. 
MALACOSTRACA. 
1. Phyllocarida. 
The oldest malacostracans whose appendages are known are species of Hymenocaris. 
One, described as long ago as 1866 by Salter, has what seem to be a pair of antenna? 
and a pair of jaw-like mouth-parts. Another more completely known species has recently 
been reported by Walcott (1912 A, p. 183, pi. 31, figs. 1-6). This latter form is described 
as having five pairs of cephalic appendages: a pair of minute antennules beside the small 
pedunculated eyes, a pair of large uniramous antenna?, slender mandibles and maxillula?, 
and large maxilla? composed of short stout segments. There are eight pairs of biramous 
thoracic limbs, the exopodites setiferous, the endopodites composed of short wide segments 
and ending in terminal claw-like spines. These appendages are like those of trilobites. 
Hymenocaris belongs to the great group of extinct ceratocarid Crustacea which are 
admitted to the lowest of the malacostracan orders, Phyllocarida, because of their resem- 
blance to Nebalia, Paranebalia, Nebaliopsis, and Nebaliclla. the four genera which are at 
present living. The general form of the recent and fossil representatives of the order is 
strikingly similar. The chief outward difference is that in many of the fossils the telson 
is accompanied by two furcal rami, while in the modern genera it is simple. It now be- 
comes possible to make some comparison between the appendages of Hymenocaris of the 
Middle' Cambrian and the Nebaliida? of modern seas. 
In both there are five pairs of cephalic and eight of thoracic appendages, while those 
of the abdomen of Hymenocaris are not known. 
In both, the antennules are less developed than the antenna?. In the Nebaliida? the 
antennules show evidence of having been originally double (they are obviously so in the 
embryo), while they are single in Hymenocaris. In both, the antenna? are simple. The 
remaining cephalic organs are too little shown by the specimen from the Middle Cambrian 
to allow detailed comparison. The mandibles, maxillula?, and maxillre of Nebalia are, how- 
ever, of types which could be derived from the trilobite. 
