DESCRIPTION OF GENERA AND SPECIES. 73 
Lower Cambrian. The outline of the valve is transversely and broadly oval in 
outline, exclusive of the apex rising above the posterior margin; the apex gives a 
subtriangular outline to the valve when looking down upon it; the apex is moderately 
elevated, and projects over the posterior margin; it is perforated by a rather large 
aperture just above a small false area. 
The surface of the valve is marked by low, rather broad, concentric undulations, 
a few fine, concentric striz, and a very finely reticulate ornamentation, formed by 
the crossing of oblique, elevated, curved lines, which form slightly elongate, diamond- 
shaped pitsbetweenthem. Shell substanceapparently calcareous. Widthof ventral 
valve, 2.5 mm.; length, 2mm.at aperture, 2.25 mm. atapex; elevation, 0.5 too.75 mm. 
As far as may be determined by the exterior of the valve this species is properly 
referred to Yorkia. ‘The generic reference, however, will remain in doubt until 
information is available as to the characters of the interior of the valve. 
Formation and Locality——Middle Cambrian: (C71) Massive cliff-making lime- 
stone in the central portion of the Ki-chéu formation [Willis and Blackwelder, 1907, 
pp. 139 and 145 (second list of fossils)], 4 miles (6.4 km.) southwest of Tung-yii, 
Shan-si, China. 
Collected by Bailey Willis and Eliot Blackwelder. 
Genus ACROTHELE Linnarsson. 
For description and discussion of genus see Monogr. U. S. Geol. Survey, vol. 11, 
I9I2, pp. 630-634. 
Acrothele matthewi eryx Walcott. 
Plate 3, Figures 6, 6a—h. 
Acrothele matthewi eryx WALCOTT, 1905, Proc. U.S. Nat. Mus., vol. xxrx, p.11. (Original description 
of variety.) 
Acrothele matthewi eryx WALCOTT, 1912, Monogr. U.S. Geol. Survey, vol. 11, p. 649, plate Lx1, fig. 2. 
(Description and illustration of variety.) 
The first identification of this species in China [Walcott, 1905), p. 11] was based 
on a single ventral valve 6 mm. in diameter. In the J. P. Iddings collection from 
southern Manchuria there is a fine series of specimens showing many characters 
of both the ventral and dorsal valves. These, when compared with Acrothele mat- 
thewi (Hartt) [Walcott, 1912b, pp. 647-649, plate LXxI, figs. 1, ra~g], indicate only 
a varietal difference between the widely separated forms. In fact, if the Asiatic 
and American specimens were all from the same locality and layer of rock, it is 
doubtful if I would separate one from the other even as a variety. ‘The illustra- 
tions of the American shells [Walcott, 1912b, plate Lx1, figs. 1, 1a—i] and the Asiatic 
shells [plate 3 of this paper] afford the student the means of comparison of specimens 
from the two continents. 
The shell is built up of several layers of lamelle that are smooth and shiny on 
the interior, except where slightly roughened by slight vascular markings and obscure 
radiating strie. The outer surface is dull and marked by concentric striz and lines 
of growth, and numerous fine, irregular, often anastomosing, elevated lines that 
give the surface a rough appearance. 
Most of the valves are slightly broader than long. The largest ventral valve 
in a limestone matrix has a length of 7.5 mm.; width, 8.5 mm.; and the apex is 1.5 
mm. in advance of the posterior margin; a large dorsal valve has a length of 11 mm.; 
width, 12.5 mm. 
Formation and Locality—Middle Cambrian: (C4) Limestone nodules at the 
base of the lower shale member of the Kiu-lung group [Blackwelder, 19074a, pp. 37 
and 4o (second list of fossils), and fig. 10 (bed 4), p. 38], 3 miles (4.8 km.) south- 
west of Yen-chuang, Sin-t’ai district, Shan-tung, China. 
Collected by Eliot Blackwelder. 
