macombi Meek 1876, found in the middle of the Juana Lopez Member
of the Mancos Shale. This specimen is 335mm dia. These are
probably a new species because the shell on typical P. macombi
usually becomes smooth at some later stage, these have strong fold like
ribs throughout growth, intermediate between P.
macombi and P. wyomingensis, they also have the beginings
outer row of ventro-lateral tubercles.