DESCRIPTION OF GENERA AND SPECIES. 227 



This species differs from Alula flexilis Matthew [1896, p. 198] in having a 

 straight hinge-line; in this respect it resembles some forms of Leperditia. For 

 the present it is referred to Alula on account of its close resemblance to Alula 

 sterope. 



The specimens were collected from a fragment of compact, bluish-gray, thin- 

 bedded limestone containing fragments of a trilobite that suggests Dorypyge. 



Formation and Locality. Middle Cambrian: (C32) A fine-grained bluish-black 

 limestone bowlder believed to have come from the lower part of the Ki-sin-ling 

 limestone [Blackwelder, 1907^, p. 272]; collected in river drift i mile (1.6 km.)south 

 of Chon-p'ing-hien, on the Nan-kiang River, southern Shen-si, China. 



Collected by Bailey Willis and Eliot Blackwelder. 



Aluta sterope (Walcott). 



Plate 23, Figure 14. 

 BradoriasteropeVfALCort, 1905, Proc.U. S. Nat. Mus., vol.xxix, p. 101. (Species described as below.) 



Outline of shell obliquely semicircular. Hinge-line straight, nearly as long as 

 the width of the valve ; anterior cardinal angle about 80 ; anterior curve obsolete ; 

 from the anterior cardinal angle the margin slopes downward and slightly inward, 

 curving gently into the broadly rounded lower margin; posterior cardinal angle 

 slightly obtuse; posterior margin curves gently from the angle to the broad curve 

 of the lower side of the valve, which gives a broadly rounded posterior end. The 

 valve is rather strongly convex, rising to the greatest height near the center. The 

 surface is marked by a very narrow rim ; from the anterior cardinal angle a narrow 

 ridge extends obliquely inward about one-half the distance toward the center, and 

 terminates in a slight tubercle ; on the anterior side there are three shallow depres- 

 sions, as though the surface had been indented; on the posterior side there is one 

 larger depression directly back of the tubercle at the end of the ridge, and a slight 

 depression in the angle formed by the palpebral ridge, the hinge-line and the ridge 

 between the two depressions. 



Surface with minute scattered punctae, as seen under a strong lens. 



The valve has a width of 1.125 mm. ; length, 0.8 mm. ; depth, about 0.25 mm. 



In outline this species resembles Aluta fragilis [p. 226]; it differs in its stronger 

 shell and distinctly marked ridge and ocular tubercle. 



Formation and Locality. Middle Cambrian : (C 32) A fine-grained bluish -black 

 limestone bowlder believed to have come from the lower part of the Ki-sin-ling 

 limestone [Blackwelder, 1907(7, p. 272] ; collected in river drift i mile (1.6 km.) south 

 of Chon-p'ing-hien, on the Nan-kiang River, southern Shen-si, China. 



Collected by Bailey Willis and Eliot Blackwelder. 



Aluta woodi (Walcott). 



Plate 23, Figure 15. 

 Bradoria woodi WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, p. 102. (Species described as below.) 



Outline of shell obliquely semicircular. Hinge-line straight, a little shorter 

 than the greatest width of the shell; anterior cardinal angle about 70; posterior 

 cardinal angle obtuse ; the posterior margin has a gentle curvature from the angle, 

 which gives it a broadly rounded outline down to where it merges into the broadly 

 rounded lower side; anterior margin almost straight and then gently curving into 

 the lower margin. Surface rather convex, with the highest portion at the ridge 

 around the central depression; the outer rim is very narrow and slightly rounded; 

 from the anterior cardinal angle a narrow, sharp ridge extends obliquely inward 



