JONES. — A REVISION OF THE GENUS ZEXMENIA. 145 



may well be called § Auc/tenocarpa, a name which indicates the achenial 

 character common to all these species, namely, a more or less marked 

 constriction of the achene near its summit, forming a sort of neck, which 

 separates the insertion of the pappus from the body of the achene. The 

 awns in this section are generally fragile and pubescent. Squamellae 

 are present in most of the species but are usually quite distinct. The 

 body of the achene varies in the differeut species from the type found in 

 § Euzexmenia on the one hand to 4-angled in the ray-flowers and 3- 

 angled in the disk-flowers, or somewhat rounded in both. In none of the 

 species of § Auchenocarpa do we find the rather dense inflorescence of 

 many species of § Euzexmenia, the heads being commonly solitary on 

 long leafy stems, or iu few-headed cymes or more rarely somewhat 

 corymbose. 



So far as known Zexmenia is strictly a New World genus and is con- 

 fined to the tropical and subtropical regions of the mainland. Three 

 species are found in the soutli western part of the United States and four 

 in South America. The greater number, however, are confined to 

 Mexico and Central America. 



This revision has been prepared at the Gray Herbarium, under the 

 direction of Dr. B. L. Robinson, where it has been possible to study 

 what is probably the largest collection of this particular group. The 

 material examined, however, has also included that of the United States 

 National Museum, the herbarium of Mr. John Donnell Smith, and of 

 the Missouri Botanical Garden. Acknowledgment is due to Mr. F. V. 

 Coville and Dr. J. N. Rose of the National Museum, Mr. Smith of 

 Baltimore, and Professor Trelease of the Missouri Botanical Garden, for 

 their obliging loan of these specimens. Through the kindness of Sir 

 W. T. Tinsel ton Dyer and Mr. W. Botting Hemsley of the Royal 

 Gardens at Kew, and Mr. Casimir de Candolle of Geneva in sending 

 type-material and tracings, several species have been identified which 

 would otherwise have remained indefinite. Special thanks are due to 

 Dr. B. L. Robinson, Dr. J. M. Greenman, and Miss Mary A. Day for 

 their assistance in the preparation of the paper, the revision of the manu- 

 script, and the proof-reading. 



§ 1. EUZEXMENIA. Perennial herbs or shrubs, with simple or 

 branching stems, generally pubescent: leaves all opposite, more or less 

 serrate, short-petioled or sessile : peduncles solitary or the heads corym- 

 bose or borne in umbelliform clusters ; heads never more than 2 cm. lou°- ; 

 involucre subglobose, campanulate, or short-cylindric, the scales 1-5- 

 seriate: ligules usually conspicuous, yellow (except in Z. zinnioides) ; 

 vol. xli. — 10 



