FORMATIONS AND FOSSILS OF SOUTH MANCHURIA 51 



Formation and locality. — ^Lower Ordovician, Wuting formation: 

 Black banded limestones near Tai-chia-pu-tzu, 2 miles west of Chiao- 

 tou station, Liao-tung, Manchuria, China. 



Genus OXYDISCUS Koken, 1889 



OXYDISCUS PLANULATUS, new species 



Plate 23, Figuees 11, 12 



A single rather good specimen constitutes the material on whicli 

 this new species is based. 



Description. — Shell flat ; dorsum acutely carinated ; greatest diam- 

 eter 25 mm ; greatest thickness or width about 5 mm. Volutions four 

 and one-half, thickest near umbilicus, from which the surface de- 

 scends by a rather flat slope to the sharp periphery, each volution 

 embracing one-third of the preceding one ; umbilicus exposing all the 

 whorls, its width about one-half of the diameter of the shell ; edge of 

 umbilicus very obtuse. Aperture not observed. Surface marked by 

 fine and distinct lines of growth which curve strongly backward 

 toward the peripheral carina. 



This species is distinguished quite easily from the other species 

 of Oxydiscus by its very flat shell and large shallow umbilicus. 



Formation and locality. — ^Lower Ordovician, Wuting formation: 

 In the black banded limestomes just north of Tai-chia-pu-tzu, 2 miles 

 west of Chiao-tou station, Liao-tung. 



Eolotype.—U.S.'^.M.. No. 83648. 



Genus RAPHISTOMA Hall, 1847 



RAPHISTOMA cf. AEQUILATERUM Koken? 



Plate 23, Figures 14-18 ; Plate 27, Figiires 1, 2 



1911. RapMstoma cf. aequilaterum Koken, Freeh, iri von Richthofen's China, 

 vol. 5, p. 12, pi. 3, figs. 6a-8b. 



Von E-ichthofen, on his well-known Chinese expedition, collected 

 two specimens of RapMstoma at Hsiau-sorr (Hsiao-shih?) along the 

 Tai-tzu-ho River, which were later identified by Freeh as Raphis- 

 toma cf. ojeguilaterum Koken. I collected a number of specimens 

 of Raphistomu (U.S.N.M. Nos. 83649-83651), which, judging from 

 the published figures and description, are close to this form. 



Description. — Shell of medium size, consisting of about four volu- 

 tions which at first enlarge gradually then rapidly in the last whorl. 

 Whorls very gently convex; umbilicus wide with depressed side; 

 section of whorl lenticular and with such a sharp peripheral angle 

 that the marginal carina is scarcely perceptible. The successive 

 whorls embrace the lower half of each inner one. A medium-sized 



