ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 53 



small but distinct, rounded, located a little nearer the hinge line than 

 the anterior extremity; from base of node a delicate ridge extends 

 toward the antero-dorsal angle. Dorsal depression not well defined, 

 small, confined to the area above the node and ridge. Marginal 

 rim clearly defined, of moderate width, thick, flattened on the inner 

 and outer faces. Except for a few shallow scattered pits that may 

 be accidental, the surface of the shell appears smooth under a hand 

 lens. Under a high power of magnification it shows exceedingly 

 minute punctae. 



Dimensions: Length of valve 1.8 mm., greatest height 1.00 mm., 

 depth about 0.5 mm. 



Remarks. — The unique left valve on which this species is based is 

 readily distinguished from all the other species referred to the genus 

 providing its present form is normal for the species. The fact that 

 the dorsal edge of the valve is turned strongly inward so that in a 

 view of the posterior edge the inner fine of the profile is concave 

 instead of straight, is highly suggestive of distortion. If the present 

 form of the valve is normal, then the concavity of the edges indicates 

 a wide gap between the valves such as is entirely unknown in the 

 otherwise similar species with which A. bergeroni is associated. If, 

 on the contrary, it is admitted that the extraordinary incurvature 

 of the back is due to distortion, it will be found, when the easily 

 effected restoration of the original form has been made, that the 

 normal outline is almost the same as in yl. eris and A. stereope. With 

 the hmited material available for comparison, it is impossible to 

 estabhsh either of these two suppositions. Under the circumstances 

 it would be unfair to take a more decided stand than to say that the 

 latter interpretation is thought the more hkely to accord with the 

 facts. Further, it should be said that the normal form of A. bergeroni 

 is probably more nearly hke that of A. eris and A. stereope than is 

 indicated by the figures of the three forms given on Plate 8. Finally, 

 a second specimen, unfortunately imperfect anteriorly, agrees closely 

 in all its remaining parts with the probable normal form of A. berg- 

 eroni. 



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

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

 Chen Ping Hsien, southeastern Shensi, China. 



Flolotype.—C&t. No. 56493, U.S.N.M. 



ALUTA ERIS (Walcott) 



Plate 8, Figure 1 



Bradoria eris Walcott, Proc. U. S. Nat. Mus., vol. 29, 1905, p. 100. 

 Aluta eris Walcott, Research in China, Carnegie Institution, vol. 3, 1913, p. 

 226, pi. 23, fig. 12. 



Description. — Carapace obliquely acuminate subovate, broadly 

 rounded posteriorly, narrowing anteriorly; hinge about three-fourths 



