123 
meris 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 frag- 
mentary condition, it will be useful to construct a key based upon 
such characters as are always likely to be present, even if specific dis- 
tinctions are not always reached. In many cases, species are 80 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 dis- 
crimination 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 dis- 
tinction between Humamillaria 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 tuber- 
cles in drying develop folds which simulate grooves, and especially is 
this true in quadrangular tubercles. In such eases 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:o the serial num- 
bers of the synoptical presentation Forms occurring within the United 
States are italicized: 
I. Tubercles never grooved. 
*Central spines none. 
Radials 5 to 9, stout. 
meiacanthus (7). 
Radials 20 to 40. . 
micromeris (12), greggii (18). 
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), 
