Crinoid grains from Mississippian skeletal packstones and grainstones from the Lake Valley and Kelley Formations, New Mexico, have retained most of their original Mg, which now resides in 1 to 10 mu m microdolomite inclusions. The mean microdolomite content of these crinoids is about 9.3%+ or -1.1% (95% confidence limit) by volumes, as determined by petrography. The microdolomite contents are lowest in the northern part of the study area (about 6%), and highest (about 11%), to the south. Original Mg contents of Mississippian crinoids were comparable to those of modern crinoids and that the relatively low mean Mg content of Mississippian crinoids is due to diagenetic loss of Mg. Mg loss from Mississippian crinoids is supported by the decrease in microdolomite content toward rims of grains, and by the fact that intragranular variation is lower than intergranular variations of microdolomite content. On average, Mississippian crinoids have retained about 75% of their Mg. The southward increase in Mg content of Mississippian crinoids we interpret to reflect a greater diagenetic Mg loss in the north than in the south. This interpretation is supported by the southward decrease in intensity of chemical compaction and by the covariance of Mg and delta 18 O contents of crinoids. Our diagenetic model, based on these data and on previous work, invokes regional, fresh, phreatic groundwater systems that formed in response to late Misssissippian-early Pennsylvanian emergence of the shelf. The groundwaters were recharged in the north and flowed, generally, southward. Northern areas contained diagenetically more aggressive ground-waters which leached more Mg from crinoid grains than did groundwaters farther south.--Modified journal abstract.