786 ARTHROPODA phylüm vii 



which bear gills ; the last has a central spatulate process that, comhined with 

 swimmerets, forms a strong caudal fin. 



It is doubtful whether this group, which in the biramous jointed legs and 

 Compound telson possesses crustacean features, belongs to the merostomes or 

 connects the latter with the crustaceans. 



Walcott, from his discovery of jointed body appendages in Sidneyia, is 

 inclined to the view that this genus is transitional between trilobites and 

 Eurypterids. One family is recognised, the original diagnosis of which is 

 given as follows : 



Family 1. Sidneyidae Walcott. 



Cephalothorax small, without lohes, eyes marginal; ventral side with large 

 epistoma, five pairs of movahle appendages, the gnathohases of the three posterior 

 pairs forming organs of manducation. Abdomen twelve-jointed, the three posterior 

 Segments annular and narrow, the terminal one forming, with lateral swimmerets, a 

 fan-like tail ; nine anterior segments with a pair of branchial appendages on each ; 

 the three posterior segments without ventral appendages. Surface smooth, or 

 ornamented by narrow, irregulär, fine imbricating ridges. 



In this family are placed two genera, Sidneyia and Amiella Walcott, from 

 the Ogygopsis shale of the Stephen formation (Middle Cambrian), near Field, 

 in British Columbia, Canada. They are described and figured in Smithson. 

 Mise. Coli., 1911-12, vol. Ivii., nos. 2 and 6. Sidneyia inexpectans Walcott, 

 the type species, which attains a length of about 17 cm., is represented by 

 very fine material. The accompanying genus Amiella is less satisfactorily 

 preserved, and there are indications of its occurrence also in the Cambrian of 

 Yunnan, in Indo-China. 



[The foregoing «chapter on the Merostomata has been revised for the present edition by 

 Dr. John M. Clarke, New York State Geologist, and drawings for several new figures of 

 Eurypterids have been kindly furnished by hini and Dr. R, Ruedemann, ofAlbany.— Editor.] 



Subclass B. EMBOLOBRANCHIATA Lankester.' 



Arthropods with at hast three preoral segments in the adult steige, ivith one pair of 

 preoral appendages called chelicerae, and five postoral pairs, the anteriormost of ivhich 

 are the pedipalpi. Chelicerae two- or three-jointed, retrovert or chelate. Pedipalpi 

 pediform, chelate or retrovert, typically six-jointed, legs typically seven-jointed. Uead 

 fused with at hast one thoracic segment, usually with the entire thorax, forming a 

 cephalothorax or prosoma. Genital opening on the first somite of the mesosoma. 

 Nephridia modified as coxal glands. Abdomen typically composed of twelve segments, 



^ Literature : Gomstock, J. H., The Spider Book. New York, m2.—Fritsch, A., Paläozoische 

 Arachuiden. Prague 1904.-ÄWi, C. L., and Berendt, J. C, Die im Bernstein befindlichen 

 Crustaceen, Myriapoden, Arachniden und Apteren der Vorwelt. Berlin, l^U.—Lankester, E. R., 

 Ar icles on Arachuida and Arthropoda in Encycl. Brit., IQll.-Laurie, M., Gn a Silnrian Scorpion 



tl^^ff ""^^^''^'^'/^f^^^^ozoic Arachnida of North America. Trans. Conn. Aead. Sei. (In 

 KTainTn Q im'i ^^"«g^^P^^ of the terrestrial Carboniferous Arachnida of Great Britain. 

 /./ r 3; r'-' ^l^h~n''f''' ^- ^^•' ^''''^ «Pi^^^«- H^r^^rd Univ. Bull., 1882, vol. ii.- 

 His ' «QO V r-' r?" ^f^,°"^f«^of Arachnida of North America. Mem. Boston Soc. Nat. 

 fr^l ArSids 'r;;^ tT^ P V^c.*^'" ^r^"'' ^^''^^ ^'''^'^' °f ^^^ World, including Myriapods 

 branchLta in Pnn 1 1 ^-S- ^«0 • Surv. No. 71, ISn.- Warhurton, C, Chapter on Embolo- 



r^i le Siluria^ 'n^^ A "'"'"^ ^S^^^F' ^""^^"^' ^^OQ.-Whüßek, k P. Fossil Scorpion 



trom the Silunan rocks of America. Bull. Amer. Mus. Nat. Hist., 1885 vol i 



