'Waterfall celluloid' or an imitation of it (such as in acrylic). The 'waterfall' description refers to the fine parallel lines seen in the material. Some are 'wavy' patterns, but with generally the same aesthetic seen in the depth of the material; and all seem to be in that 'ivory' color (whitish/yellowish). Lots of examples in Google Images, if searching on the 'waterfall celluloid' descriptor.
David