Z ART. <>. 3r. YOKOYAMA ON .SOME 



Professor Jimbo in his " Report of the Geological Researches 

 of Eastern Rikuzen " (M S) mentions six horizons or beds as 

 constituting the Jurassic of that region, which counted from below 

 are as follows : 



I. Cjjrena bed of 3Iaf/enosn and N'h'anohainn, a dark 

 clayslato filled with Cyrena. 



II. KaraJi'uwa Clay slate, with Gastropods, Lamellibranchs 

 and Crinoids. 



III. Irlnosawa Clayslate with EHheria-WVe shells. 



lY. Trigonia Bed, consisting of a hard, light-grey, medium- 

 grained sandstone filled with shells of Trigonia V-costata Lycett. 



V. Sandy Clayslate, with Arieliles and Belemnites. 



VI. Clayslate, with Ammonites. 



Of these six beds, he considei-s the first three as the lower, 

 and the other three as the upper part of the formation separated 

 l)y a line of unconformability. 



The subdivisions subsequently proposed by Iki are similar to 

 the above, although not quite the same. 



The fossils described bolow are those obtained in the beds 

 I., IV., v., and VI. They are the following : 



From Bed I. 



1. Cyrena hnivlata n. sp. 



2. Cyrena ohlonga n. sp. 



13. Trigonia hosourensis n. sp. 



4. Perna rikuzenica n. sp. 



5. Gervillia trigona n. sp. 

 G. Baiera f sp. 



From Bed lY. 

 1. Trigonia V-costata Lycett. 



