ABT. 4 CAMBRIAN CONCHOSTRACA — TTLRICH AND BASSLER 55 



headed small elevation close to the middle of the dorsal edge. Mar- 

 ginal rim narrow but sharply defined and uniform in development. 

 Surface of shell very minutely punctate, appearing smooth under an 

 ordinary hand lens. 



Dimensions of type, a left valve: Length 1.0 mm., height about 

 0.75 mm., depth about 0.25 mm. 



Remarks. — Only a single, well preserved left valve of this species 

 has been found. It is the smallest of all the known Cambrian 

 bivalved Crustacea. Perhaps it is a young example. Assuming 

 that it is mature and retains its normal characteristics, the species is 

 readily distinguished from the preceding A. eris and A. bergeroni by 

 its more quadrangular shape. The former species is further differen- 

 tiated by having a third (antero-ventral) tubercle, while in A. berger- 

 oni the antero-dorsal depression is less conspicuous, and both are 

 represented by larger specimens than A. enyo. The type of A. stereope 

 is similarly of small size and is thought to be its closest ally. Indeed, 

 the most striking difference between the types of A. enyo and A. 

 stereope, namely, certain oval depressions in the anterior half of the 

 latter that are not seen in the former, may really be due to accidental 

 causes. 



Occurrence. — Middle Cambrian; compact, bluish gray, thin-bedded 

 limestone; from shingle on gravel bar in the Lan Ho, 1 mile south of 

 Chen Ping Hsien, southeastern Shensi, China. 



Holotype.— Cat. No. 56497, U.S.N.M. 



ALUTA STEREOPE (Walcott) 



Plate 8, Figures 9, 10 



Bradoria stereope Walcott, Proc. U. S. Nat. Mus., vol. 29, 1905, p. 101. 

 Alula stereope Walcott, Research in China, Carnegie Institution, vol. 3, 1913, 

 p. 226, pi. 23, fig. 14. 



Description. — Carapace small, obliquely acuminate-subovate in 

 outline, the rectangular antero-dorsal part suggesting the quadran- 

 gular; hinge straight, about three-fourths as long as the valve, 

 terminating sharply in front, less abruptly behind; posterior and 

 ventral sides rounded. Surface of valve rather strongly convex, 

 rising to the greatest height near the middle. A thin ridge, beginning 

 at the antero-dorsal angle, extends inward about half the distance to 

 the middle of the valve, where it terminates in a small tubercle. 

 Four dentlike depressions, arranged somewhat symmetrically with 

 respect to the ridge, and possibly of accidental origin, occur in the 

 antero-dorsal half of the valve. One of these lies in the triangle 

 between the ridge and the dorsal edge, a second, large and elliptic in 

 shape, stretches half the distance from the tubercle to the postdorsal 

 angle. The other two depressions occupy most of the space between 

 the lower side of the ridge and the anterior margin. Marginal rim 



