1915] 



BRITTON VEGETATION OF MONA ISLAND 35 



(Mamillaria nivosa) being more plentiful here than on any 

 other island visited by us; Opuntia Taylori, hitherto known 

 from Hispaniola, Culebra and the Virgin Islands, was found as 

 a single colony; this has not yet been detected on the Porto 



Rican mainland. 



The limestone plateau of Mona is bordered nearly through- 

 out by steep escarpments and is accessible at but few points, 

 except along the southwestern side, where there is a low plain 

 several miles long and averaging about half a mile wide, from 

 which the plateau is reached at a number of points over a 

 talus of large limestone blocks. At the foot of the escarpment 

 and of the talus on this southwestern side, the moistest condi- 

 tions of Mona occur, and several species of trees here reach 

 large size, notably the manchioneel (Hippomane Mancinella) 

 and two species of Ficus. Here also grow two species of 

 ferns, several bryophytes, and a number of Polyporaceae 

 infesting dead wood. The soil of the narrow plain is more 

 abundant than that of the plateau, permitting agricultural 

 operations on a small scale and supporting a low forest made 

 up of a considerable number of kinds of trees, with more her- 

 baceous vegetation than exists on the plateau. Among rare 

 elements of this vegetation are two orchids, Domingoa hyme- 

 nodes, hitherto known from Hispaniola and Cuba, and Ibidium 

 lucayanum, of Porto Rico, Anagada and the Bahamas. The 

 coastal sands, which extend almost uninterruptedly along the 

 shore of the plain, are inhabited by characteristic West Indian 

 sand-dune species. 



Lichens are quite abundant on tree trunks and on rocks of 

 the talus, including a considerable number of species. Pro- 

 fessor Lincoln W. Riddle has examined the collection and has 

 submitted the following report upon them : 



"The exploration of Mona Island has yielded 42 numbers of 

 lichens, 40 collected by Dr. N. L. Britton, Messrs. J. F. Cowell and 

 W. E. Hess, and 2 collected incidentally by Dr. F. L. Stevens. These 

 42 numbers represent 26 species in condition for determination. 



"The species growing on the limestone rocks constitute the most 

 striking and interesting part of the collection. These include four 

 species of Omphalaria, a species of Collema, and a species of the Der- 

 matocarpaceae, which is, unfortunately, sterile and, therefore, not 

 further determinable. The omphalarias are all little known species. 



