April, 1916. Alacran Reef — Millspaugh 425 



Two shrubs only had secured a footing on this cay, one each of Conocar- 

 pus and Toumefortia. Scattered amongst the dense growth of Sesuvium 

 was found a number of clumps of the new Cenchrus, forming its only 

 habitat, and one climip each of the new Tribulus, Portulaca oleracea 

 and Atriplex cristata. In the clear sand beyond the Sesuvium at its 

 northernmost margin was another colony of the new Cenchrus. 



Chica (Little) Island is another low, pure sand cay, fusiform in 

 outline, 575 x 325 feet in extent. 



On this islet the disposition of the Sesuvium and Sporobolus areas 

 is reversed in comparison with the other two cays, while the scattering 

 other species are interspersed with the Sporobolus colony instead of the 

 Sesuviiun. In this the clothing of the islet is quite similar to that of the 

 foot of Perez and shows plainly that it forms a resting place for web- 

 footed birds. Here the new Tribulus, Atriplex cristata and the Chamae- 

 syce grow in profuse masses while the new Cakile maintains its charac- 

 teristic habitat on the weather beach line. 



Allison Island, three rmles from the other cays described, differs in 

 several respects from the other three. First : it lies east and west — the 

 others essentially north and south; second: its entire coast line is weather 

 shore; i. e., abrupt and eroded above a narrow strand, except a small 

 portion of its western extremity which is somewhat spit-like; third: 

 it is considerably higher (about 8 ft.) and flat Hke a table-land. Its 

 dimensions are about 1000 x 275 feet. 



Notwithstanding the fact that this islet is comparatively large it 

 nevertheless gives place to but six plant species. Its "grass " and "sam- 

 phire " colonies are pure, as on the other islets. The largest bird colonies 

 of all are on this cay, in fact it is almost wholly a nidulation ground. 

 Boobies appropriate the grass colony and frigate-birds the Sesuvium 

 tract. It is on this cay that Tribulus alacranensis evolved, its presence 

 on the other cays being a matter of transplantation through its burs 

 having adhered to the feet of the booby. The species exists in dense 

 colonies amongst the Sporobolus, and it is of its ligneous stems and 

 branches that the boobies build their nests. The only colony of Philox- 

 erus vermicularis on the cays is here in the Sporobolus, as well as a widely 

 scattering growth of the Chamaesyce. Again the new Cakile lines 

 itself along the weather strand but only on the weathermost side. 



Utowana Cay, which we sighted but were unable to attain, is the 

 northernmost islet of the group. Through the glass its surface appeared 

 very like that of Allison Island. As it lies north and south much 

 further within the bank of the reef than any of the other cays its vegeta- 



