114 PALEOZOIC PALEONTOLOGY. 
are not at all as in that genus. The muscular impression of the 
pedicle valve has some more or less remote resemblance to that of 
members of the typical genus Orthis, but the impression in the brachial 
valve is quite different. Doubtless there are other similar Cambrian 
Orthids, but for the present, until all the Cambrian brachiopods have 
been thoroughly investigated, the species may be placed provisionally 
in the genus Orthis. 
ARTHROPODA. 
TRILOBITA: 
MICRODISCUS? sp. undet. 
Plate hie, Bic at: 
A single specimen, too imperfect for identification or description, 
appears to belong either to Microdiscus or Agnostus. The specimen 
is probably a head, and is nearly semi-circular in outline, being pro- 
portionately much broader than is usually the case with members of 
the genus Agnostus, for which reason it is placed provisionally in 
Microdiscus. 
; 
OLENELLUS THOMPSONI (Hall). 
Plate II., Figs. 9-10. 
1859. Olenus thompsoni Hall, Pal. N. Y., vol. IIL, p. 525, fig. on 
p. 526. 
1884. Olenellus thompsont Whitfield, Bull. Am. Mus. Nat. Hist., 
vol. I., p. 151, pl. 15, figs. 1-4. 
1886. Olenellus thompsoni Walcott, Bull U. 8. Geol. Surv., No. 
30; p. 167, pl. 17, figs. 1, 2, 4, 9, pl 22, fig: 1, pl: 23, fea 
1890. Olenellus thompsoni Walcott, 10th Ann. Rep. U. 8. Geol. 
Surv., pt. I., p. 635, pl. 82, figs. 1-1 a, pl. 83, figs. 1-1 D. 
Description.—Head, without the genal spines, sub-semi-circular in 
outline. Glabella elongate, with subparallel sides, depressed convex, 
marked by four pairs of lateral furrows. of which the anterior pair 
