24 THE CACTACEAE. 
name, however, comes from Engelmann, who first used it as a subgenus of Mammiullaria 
(Proc. Amer. Acad. 3: 264. 1856). 
The position of this group has always been puzzling to cactus students. Dr. Poselger 
believed that it was a section of Echinocactus and transferred certain of these species which 
had been described under Mammillaria to Echinocactus. In its vertical, nearly central 
flowers it does approach the Echinocactanae, but otherwise it is quite distinct. 
In the origin of their large flowers, in the shape and structure of their fruit, and in the 
color and form of their seeds the species compose a rather natural group, but they are 
diverse in form and armament. The species are most common in central Mexico, a few 
extending into the southern United States, and one extending into southern Canada. 
The groove on the upper side of the tubercle which is so characteristic of the genus 
does not occur on seedlings or on very young plants, but it is always found on old flowering 
plants and seems to be associated with the inflorescence, for the flowers appear only in the 
axils of grooved tubercles and originate at the bottom of this groove. Plants which grow 
in conservatories for a long time without flowering lose this groove; * we have had one 
plant of this kind under observation for fifteen years. 
KEY TO SPECIES. 
A. Seeds brown, not angled; flowers usually large. 
: B. Tubercles grooved to middle or a little below; ovary bearing scales with woolly 
axils. Series Macromeres. 
Tubercles elongated, bright green......... 0.0... cece cece ec eee nce ueenees 1. C. macromeris 
Tubercles short, grayish green........ 0... cee eee cee cece cece neeucuneens 2. C. runyontt 
BB. Tubercles grooved from tip to base except in young plants; ovary naked. 
C. Grooves of tubercles bearing large yellow or red glands. Series Recurvatae. 
Flowers white......... 0... ccc cece cece cee nec e ee eeencecteeneeenes 3. C. ottonts 
Flowers not white. 
Stems globular. 
s Radial spines more or less recurved............. 0.0... eceeecaceees 4. C. recurvata 
Radial spines spreading or ascending. 
Spines dark, sometimes black. ..........0 0... cece cece ccc eeues 5. C. poselgeriana 
Spines yellow or sometimes tinged with red. 
Central spines slender and flexible................ 0.00 cceeecee 6. C. muehlenpfordtit 
Central spines stout and rigid. 
Radial spines subulate...........0 0... cece ec cece ceueeue 7. C. guerkeana 
Radial spines acicular...... 0.0... cece cece cece cc ceeee 8. C. echinoidea 
Stems cylindric. 
Stems bluish green.......... 0.0... cece ccc cece cee cece eeccuceee 9. C. clava 
Stems yellowish green. 
Central spine usually one. 
Glands in groove red.......... 0... ccc cece cece cence cecceecee 10. C. octacantha 
Glands in groove yellow.........000 0... c cc cece cece ececcceces 11. C. exsudans 
Central spines 2........ 0.0... ec c ccc cece cece ce cece ece cee, 12. C. erecta 
CC. Grooves of tubercles without large glands. Series Sulcolanatae. 7 _ 
D. Outer perianth-segments not ciliate. 
E. Flowers purplish or rose.......... 0... c cee cececccecccccccey 13. C. elephantidens 
EE. Flowers yellow or white. 
F. Tubercles very large, broader than high.................... 14. C. bumamma 
Lee eee eee cece cence tence eeee 15. C. robustispina 
GG. Plants much smaller than in C, robustispina; seeds 2 mm. ° 7 
in diameter or less. 
H. Central spines usually wanting. 
Secondary cluster of spines developed in upper part of 
eee eee ee ee eee 16. C. connivens 
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Spines not pectinate. 
Spines 14 or more, 
Spines slender with long black tips........, 18. C. nickelsae 
Spines rather short with light tips. 
spines subulate.................0..000.. 19. C. compacta 
_ pines acicular.................. 
Spines fewer than 1g 20. C. radians 
Spines slender and weak................... 21. C. sulcolanata 
Spines not slender........................ 22. C. retusa 
* Mammillaria potosiana and M. polymorpha seem to have been based on such plants. 
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