and ‘Tobago have added somewhat to our knowledge of 
the range and variability of the genus, but there still re- 
mains much to clarify. 
In preparing the orchid section for the Fora of Trini- 
dad and Tobago, 1 have had to consider critically the 
concepts which have been known as Diacrium bicornutum 
and Diacrium indivisum. A study of the available her- 
barium specimens and of the very superior material pre- 
served in alcohol and sent in recently by Dr. Wilbur G. 
Downs and Dr. T. H. G. Aitken of Port-of-Spain, Trin- 
idad, made it early apparent that the fundamental prob- 
lems involved could not satisfactorily be handled without 
an examination of the generic concept as a whole. The 
present paper embodies the results of that examination. 
I wish to thank the officials of the following herbaria 
for making available material entrusted to their care: 
Reichenbach Herbarium (in the Naturhistorisches Mu- 
seum in Vienna); Gray Herbarium; Royal Botanic Gar- 
dens at Kew; New York Botanical Garden: United 
States National Herbarium; Chicago Natural History 
Museum and Missouri Botanical Garden. Material from 
these herbaria has supplemented the large collection of 
Caularthron preserved in the Orchid Herbarium of Oakes 
Ames of the Botanical Museum of Harvard University. 
It is, furthermore, a pleasure for me to thank Mr. G. C. 
K. Dunsterville for kind permission to reproduce two 
carefully executed drawings prepared for his forthcoming 
book of illustrations of Venezuelan orchids. 
When Bentham described Mpidendrum bicornutum in 
1834 on the basis of material from Trinidad, he stated 
that he had consulted Lindley concerning its generic 
status, and had received the opinion that it ‘‘is certainly 
anew species; but I think it cannot be separated from 
Kpidendrum. The only distinction between it and that 
Genus consists in the labellum being distinct from the 
[78 | 
