786 ARTHROPODA phylum vri 



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

 swimmerets, forms a strong caudal Jin. 



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 lobes, eyes marginal; ventral side with large 

 epistoma, five pairs of movable appendages, the gnathobases of the three j^osterior 

 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, irregular, 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. 

 Misc. Coll., 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 

 Euryjiterids have been kindly furnished by him and Dr. R. Ruedemann, of Albany. — Editor.] 



Subclass B. EMBOLOBRANCHIATA Lankester.' 



Arthropods with at least three preoral segments in the adult stage, xoith one pair of 

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

 are the pedip)alpi. Chelicerae two- or three-jointed, retrovert or chelate. Pedipalpi 

 pediform, chekite or retrovert, typically six-jointed, legs typically seven-jointed. Head 

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

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

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



* Literature : Gomstock, J. H., The Spider Book. New York, 1912. — Fritsch, A., Palaozoische 

 Arachniden. Prague, 1904.— /^ocA, C. L., and Berendt, J. &, Die iui Bernstein befiudliclien 

 Crustaceen, Myriapoden, Arachniden und Apteren der Vorwelt. 'Berlin, 1854. — Lankestcr, E. R., 

 Articles on Arachuida and Arthropoda in Encycl. Brit., 1911. — Laurie, M., On a Silurian Scorpion 

 from the Pentland Hills. Trans. Roy. Soc. Edinb., 1899, vol. x.x.xix. — Petrunkcvitch, A., 

 Monograph of terrestrial Palaeozoic Arachnida of North America. Trans. Conn. Acad. Sei. (In 

 press.) — Pocock, R. /., Monograph of the terre.strial Carboniferous Arachnida of Great Britain. 

 Palaeontogr. Soc, 1911. — Scudder, S. H., Fo.ssil Spidens. Harvard Univ. Bull., 1882, vol. ii. — 

 Idem, Illustrations of the Carboniferous Arachnida of North America. Mem. Boston Soc. Nat. 

 Hist., 1890, vol. iv. — Idem, Index to the known fossil Insects of the World, including Myriapods 

 and Arachnids. Bull. U.S. Geol. Surv. No. 71, 1891. — Warhvrton, C, Chajiter on Embolo- 

 brancliiata in Cambridge Natural History. London, 1909. — ■Whitjicld, R. P., Fossil Scorpion 

 from the Silurian roJks of America. Bull. Amer. Mus. Nat. Hist., 1885, vol. i. 



