MALACOSTRACA. 1 1 3 



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 antennae 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 maxillae 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 antennas disappear. The mandibles, 

 maxilluhe, and maxillae 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 setae 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 Hymcnocaris. 

 One, described as long ago as 1866 by Salter, has what seem to be a pair of antennae 

 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 antennae, slender mandibles and maxilluhe, 

 and large maxilhe 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. 



Hymcnocaris 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 Ncbalia, Parancbalia, Ncbaliopsis, and Nebaliella, 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 Hymcnocaris of the 

 Middle Cambrian and the Nebaliidas 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 antennae. In the Nebaliidae the 

 antennules show evidence of having been originally double (they are obviously so in the 

 embryo), while they are single in Hymcnocaris. In both, the antennae are simple. The 

 remaining cephalic organs are too little shown by the specimen from the Middle Cambrian 

 to allow detailed comparison. The mandibles, maxillulas, and maxillae of Ncbalia are, how- 

 ever, of types which could be derived from the trilobite. 



