GEOGRAPHY OF CENTRAL FLORIDA 229

itudes 27' and 30'. (The other species of Aphelocomca, eight or nine in number, are all western, ranging from Texas and Idaho to Central America.)
 The dusky seaside sparrow (Passerherbulus nigresceus), although described as long ago as 1873, is still known only from marshes within a few miles of Titusville on the east coast. Chapman says of it (Handbook, p. 394): "In view of the fact that this species is abundant and that the region is in no sense isolated, but that both to the north and south there are marshes apparently similar to those it occupies, the restriction of its range to an area only a few square miles in extent makes its distribution unique among North American birds."
 Besides these well-marked local species of non-migratory birds there are several other cases in which the Florida birds differ just a little from those of the -same species farther north, as stated a few pages back, but it is hardly worth while to mention them in a work of this kind.
 Aniong extinct birds there is one noteworthy record, the finding of bones identified as belonging to the great auk (Pla uts impennis) in a shell mound near Ormond by Prof. Blatchley in 1902. This penguin-like bird was chiefly confined to the colder parts of the Atlantic ocean, and there is no record of its having been seen alive since 1842.
 One avian product that deserves special mention is bird guano. The principal source of this has been a few small islands off the coast of Peru, where myriads of sea birds have roosted and nested for ages, safe from most of their enemies, and where rain is practically unknown, so that there is no leaching of the valuable fertilizing. constituents of the guano. The deposits have been exploited more or less for centuries, but the industry reached its height in the third quarter of the last century.*
 In recent years some artificial guano islands have been constructed near Cedar Keys, by building wooden platforms a few feet above the shallow waters of .the Gulf a few miles off shore.

 *Probably the most accessible descriptions of the guano islands of Peru are those by Dr. Robert E. Coker in the Proceedings of the U. S. National Museum 56:449-511, Pl. 53-69 (Sept. 1919), and in the National Geographic Magazine 37:537-566, with 28 unnumbered half-tones (June, 1920).