Crystallographic data have been accumulated for 119 fossil and Recent echinoid species. All but 3 of the 19 orders of the class have been sampled. The calcite plates of 73 species have principal optic axes which are uniformly perpendicular to the plate surface; in 35 species, the c-axes are tangent to the plate surface and aligned longitudinally on the test; 6 species have tangential c-axes in interambulacral plate columns and perpendicular c-axes in ambulacral columns; 2 species show ontogenetic variation in c-axis orientation of about 45 degrees in a single plate column (representing an age series), and 3 exhibit ontogenetic variation approaching 90 degrees . c-axis orientation patterns are constant at the species and genus levels. Only one family (Cidaridae) contains significant departure from crystallographic homogeneity, and most of the orders are homogeneous. The phylogenetic stability of calcite crystallography indicates that orientation patterns may be used as a taxonomic character and may thus contribute to knowledge of echinoid phylogeny. Several specific phylogenetic problems exemplify the usefulness of this character. The evolution of one orientation pattern from another in the Echinoidea may be related to the organism's sensitivity to sunlight, and to the amount of curvature of the individual plate, but insufficient data are available to assess these possibilities in terms of natural selection and adaptation. In any event, it is clear that the acceleration and retardation of rates of test development provide the mechanism for evolutionary change.