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 striae, and a very finely reticulate ornamentation, formed by 

 the crossing of oblique, elevated, curved lines, which form slightly elongate, diamond- 

 shaped pits between them. Shell substance apparently calcareous. Width of ventral 

 valve, 2.5mm.; length, 2 mm. at aperture, 2.25 mm.atapex; elevation,o.5 too. 75mm. 



As far as may be determined by the exterior of the valve this species is properly 

 referred to \'orkia. 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-chou 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. LI, 

 1912, pp. 630-634. 



Acrothele matthewi eryx Walcott. 



Plate 3, Figures 6, 6a-h. 

 Acrothele matthewi eryx WALCOTT, 1905, Proc. U.S. Nat. Mus.,vol. xxix. p. n. (Original description 



of variety.) 



Acrothele matthewi eryx WALCOTT, 1912, Monogr. U. S. Geol. Survey, vol. LI, p. 649, plate LXI, fig. 2. 

 (Description and illustration of variety.) 



The first identification of this species in China [Walcott, 19056, p. 1 1] 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, 19126, pp. 647-649, plate LXI, figs, i, ia-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, 19126, plate LXI, figs, i, la-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 lamellae that are smooth and shiny on 

 the interior, except where slightly roughened by slight vascular markings and obscure 

 radiating striae. The outer surface is dull and marked by concentric striae 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 1 1 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, igoja, pp. 37 

 and 40 (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. 



