Fossil nonmarine Actinocyclus species, A. nipponicus sp. nov. and A. bradburyii sp. nov., are described from Miocene lake sediments in the Yamato Bank, the central part of the Japan Sea. Actinocyclus nipponicus is characterized by an inflated valve composed of a nearly flat valve face and an inclined mantle, a bullulate valve wall, fascicles sectioned by combinations of independent areola rows and hyaline stripes, loculate areolae with external cribra, rimoportulae at ends of hyaline stripes in the mantle, a single pseudonodulus at the valve face/mantle junction and well-developed spines. On the other hand, A. brodburyii is characterized by a noninflated valve composed of a nearly flat valve face and a very gently inclined mantle, a bullulate valve wall, loculate areolae with external cribra, fascicles sectioned by combinations of independent areola rows and hyaline stripes, rimoportulae at ends of hyaline stripes in the mantle, a single pseudonodulus within the valve face and misshapen cribra behind the pseudonodulus. In A. nipponicus, fascicles are composed of parallel areola rows in large valves, but are composed of radial areola rows in small valves. Additionally, in both A. nipponicus and A. bradburyii, changes in the number of rimoportulae (fascicles) occur as a function of valve size. Actinocyclus nipponicus and A. bradburyii are the most similar to A. trapeziformis and A. cupreus, respectively.
