58 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



are not uncommon in the limestone holding this and the other Chinese 

 species of Alula. They exhibit considerable variation in form, but to 

 what extent this is attributable to distortion can not be determined. 

 Some of them doubtless belong to one or another of the associated 

 species, all of which are of smaller diameter than the type of A.fragilis. 

 Assuming that the dimensions are fairly constant for each species, 

 only the larger examples are provisionally referred to this species. 



Compared with American species, A. jragilis appears to be closely 

 allied to the genotype, A.flexilis Matthew. Aside from the fact that 

 the Chinese species is only about half as large, it differs chiefly, so far 

 as the characters are known, in having a longer hinge. 



Occurrence. — Middle Cambrian limestone, found loose on a gravel 

 bar on the Nan-kiang River, 1 mile south of Chon-p'ing-hien, southern 

 Shensi, China. Pieces of the same limestone, found on the same gravel 

 bar, contained A. enyo, A. eris, A. bergeroni, A. stereope, and A. woodi; 

 also fragments of a trilobite suggesting Dorypyge. 



Holotype.—C&t. No. 56495, U.S.N.M. 



ALUTA TROYENSIS (Ford) 



Plate 8, Figures 15-17 



Leperditia troyensis Ford, Amer. Journ. Sci., ser. 3, vol. 6, 1873, p. 138. — Wal- 

 COTT, Bull. No. 30, U. S. Geol. Surv., 1886, p. 146, pi. 16, fig. 5. 



Aristozoe troyensis Walcott, Amer. Journ. Sci., ser. 3, vol. 34, 1887, p. 193, 

 pi. 1, fig. 8; 10th Ann. Rep. U. S. Geol. Surv., 1890, p. 628, pi. 80, figs. 2, 2a. 



Description. — Carapace large for the genus, obliquely subtriangular 

 in outline, acuminate anteriorly, rounded on the postventral side; 

 dorsal side long, slightly arcuate; anterior angle about 65° to 70°, 

 postdorsal angle more obtuse but well defined, about 115°. Surface 

 only moderately convex, without distinguishable depressions, highest 

 in the. antero ventral part where it rises into a large blunt spine or 

 conical elevation. Marginal rim wide, convex, best defined on the 

 anterior side, where a corresponding concavity on the inner side of the 

 shell produces a narrow but distinct marginal ridge on casts of the 

 interior. The latter show further that the main cavity of the shell is 

 prolonged anteriorly to the extremity of the hinge and that the dorsal 

 edge is much thicker in its anterior half than in its posterior part. 

 Shell comparatively thick and firm in the anterior part, but otherwise 

 apparently as in other species of the genus. 



Dimensions of a left valve: Greatest diameter 7.5 mm., greatest 

 height 5.0 mm., thickness about 1.5 mm. 



Remarks. — This species is readity distinguished from all the pre- 

 ceding species of the genus by its greater size, firmer shell, sub- 

 triangular form, slightly arcuate dorsum, and anteroventral node. 

 These differences might possibly warrant generic separation, but after 



