138 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



branch. The CheHcerata inckide the Xiphosurida, the Eurypterida, 

 the Arachnida, the Acarinida, and very probably the Pycnogonida. 



12. — The Xiphosurida are undoubtedly the closest living repre- 

 sentatives of the Trilobita. The xiphosurid prosoma has the same 

 structure as the trilobite head, and the same composition except for 

 the addition of three extra somites and a part of the eighth somite. 

 Likewise, the opisthosoma corresponds with the trilobite pygidium 

 extended forward to include all the somites behind the prosoma, so 

 that in the Xiphosurida there is no intermediate "thoracic" region 

 of free segments. Such fossil forms as Belinurus and Prestzvichia 

 would appear to be intermediate between modern Xiphosurida and 

 Trilobita, and the Middle Cambrian Naraoia (see Walcott, 193 1, 

 fig. I ) must be related to the xiphosurid line somewhere close to the 

 trilobites. The first six prosomatic appendages retain the leg type 

 of structure, except for the reduction and chelicerate form of the 

 first pair. The seventh appendages are reduced to a pair of small 

 lobes, the chilaria, and the following six have the form of broad 

 plates formed chiefly by epipodite lobes, those of the last five bearing 

 lamellate gills. The genital openings in both sexes are on a median 

 ventral fold of the eighth segment united with the bases of the 

 opercular appendages of this segment. 



/J. — The Pycnogonida, judging from some of their structural 

 features, such as the union of the anterior body segments, the pos- 

 terior position of the dorsal eyes between the bases of the third pair 

 of appendages, the presence of a patellar segment in the legs, and 

 the chelicerate structure of the first appendages, are to be classed 

 with the Chelicerata; but because of their many unique characters, 

 including the occurrence of multiple genital openings, it is impossible 

 to connect them closely with any other of the chelicerate groups. It 

 may be noted, however, that species with eight pairs of legs have 

 presumably the same number of somites in the prosoma as have the 

 Xiphosurida. 



/^. — The Eurypterida and the Arachnida differ from the xipho- 

 surids in having only six segments in the prosoma, and this character 

 together with various other features of their organization shows that 

 these two groups are more closely related to each other than is either 

 group to the Xiphosurida. On the other hand, the Eurypterida have 

 certain characters of the xiphosurids that leave little doubt of their 

 common ancestry with the latter, and their descent from trilobite 

 stock. The general resemblance of the eurypterids to scorpions sug- 

 gests a relationship between the two, but the theory of Versluys and 

 Demoll (1920, 1923) that the Eurypterida and Xiphosurida are 



