This species shows an interesting transition from Coryphantha to Echinocactus. The woolly groove of the Coryphantha extends from the spine-bearing areola to the axil of the tubercle, where it expands into the flower-bearing areola. In C. macromeris the groove extends only about half way down the tubercle and gives origin to the flower-bearing areola on the side of the tubercle; while in Echinocactus the flower-bearing areola becomes adjacent to the spine-bearing areola and the flower appears at the summit of the tubercle. ARTIFICIAL KEY TO THE SPECIES. It seems impossible to make a simple artificial key that will serve as a useful guide to each individual species and variety. Our knowledge of so many of the species is imperfect, that no set of characters can be applied throughout. However, as no plants are collected in such fragmentary condition, it will be useful to construct a key based upon such characters as are always likely to be present, even if specific distinctions are not always reached. In many cases, species are so closely and differently related to each other that the complete descriptions will have to be consulted to determine the differences, and in such cases the artificial key can only indicate the group. Even the full descriptions are very compact, all characters not necessary for discrimination having been eliminated. No attempt need be made to determine any species by means of the flowers alone. In most cases more or less of the plant body will be available, presenting spine and tubercle characters, and these are used in the following key. The distinction between Eumamillaria and Coryphantha, on the basis of grooveless and grooved tubercles should always be made out easily. It may be useful to suggest as a caution, however, that often tubercles in drying develop folds which simulate grooves, and especially is this true in quadrangular tubercles. In such cases it is necessary to restore the original plumpness of the tubercle by boiling, before the presence or absence of the groove can be definitely determined. The species and varieties are indicated only by their specific or varietal names in the following key, and the numbers refer to the serial numbers of the synoptical presentation. Forms occurring within the United States are marked with an "*": I. Tubercles never grooved. * Central spines none. Radials 5 to 9, stout. meiacanthus* (7). Radials 20 to 40. micromeris* (12), greggii (13). Radials 40 to 80. lasiacanthus* (10), denudatus* (11). ** Central spine solitary and not hooked. + Central spine longer than the radials. Radials 7 or 8: tubercles very long (40 to 50 mm.). longimamma (36). Radials 15 to 20: tubercles 6 to 8 mm. long. eschanzieri (21). ++ Central spine shorter than the radials. Radials 5 to 9, stout. meiacanthus* (7).