NO. 2 MIDDLE CAMBRIAN MEROSTOMATA 23 



Abdomen with twelve segments, the anterior nine of which each 

 carry a pair of branchia-bearing appendages. The next two pos- 

 terior segments, tenth and eleventh, are simple, annular rings, the 

 terminal segment or telson has a central spatulate section that, with 

 its lateral swimmeret on each side, forms a broad caudal fin. 



Surface of dorsal shield smooth. 



Genotype. — Sidneyia inexpcctans, new species. 



SiratigrapJtic range. — The stratigraphic range is limited, so far 

 as known, to a thickness of 130 feet in a dark siliceous shale form- 

 ing a part of the Stephen formation and described as the Ogygopsis 

 shale in 1908.' 



Geographic distribution. — On the slope of the ridge between 

 Mount Wapta and Mount Field north of Burgess Pass, and about 

 3800 feet above Field on the line of the Canadian Pacific Railway, 

 British Columbia, Canada. 



Observations. — Sidneyia is a most interesting type and one that I 

 should have expected to find in an Ordovician rather than in a 

 Middle Cambrian formation. It is associated with a large fauna, 

 part of which is enumerated in the list of thirty-two species listed 

 under the description of the Ogygopsis shale referred to above. The 

 stratigraphic horizon in the British Columbia Cambrian section is 

 over 6300 feet below the summit of the Cambrian series.^ 



The short, broad, cephalo-thorax, the broad elliptical abdominal 

 portion formed by the first nine segments, the elongate, narrow three 

 posterior segments, the last taking the form of a broad caudal fin, 

 all unite to give the type a scorpion-like appearance. 



The genus dififers from all the genera of the Eurypterida in the 

 form of the cephalo-thorax, smooth surface, presence of a very 

 large epistoma, non-chelate antennae, absence of a metastoma, and 

 (with the exception in Stylomirns) absence of a broad posterior 

 pair of swimming appendages ; also in the arrangement of the 

 branchiae upon the nine anterior abdominal segments and in the 

 presence of the broad caudal fin formed of the spatulate terminal 

 section and swimmerets of the twelfth segment. 



In this preliminary study and description some detail is omitted, 

 but this will be worked out and inserted in the final study of the 

 genus. 



The generic name Sidneyia is proposed in recognition of the dis- 

 covery of the type specimicns by my son, Sidney S. Walcott, in 

 August, 1910. 



^ Smithsonian Miscellaneous Collections, Vol. 53, No. 5, p. 210. 

 ^ Idem, pp. 216, 217. 



