Lower California. 29 



of the arid Lower Sonoran are kangaroo rats of the genus Dipo- 

 <loini/x, pocket mice of the subgenus Chaetodip'tis, and spermophiles 

 of the subgenera Xerospermophilus and Ammospermophilus. 



The peninsula of Lower California is a subdivision of the arid 

 Lower Sonoran Zone. Not a single genus of land mammal or 

 bird is restricted to it and but two peculiar species of mammals 

 have been described. The peculiar birds are more numerous, 

 but with few exceptions are only subspecifically separable from 

 those of neighboring parts of the United States and Mexico. 

 They may be classed in two categories: (1) Mountain forms 

 derived from the North (of Boreal or Transition origin) ; and 

 (2) lowland forms derived from the contiguous plains (of Sono- 

 ran, or in one instance subtropical, origin). As would be ex- 

 pected from the character of the country, the great majority are 

 subspecies of well-known Sonoran forms, with the addition of a 

 small number of peculiar species belonging to Sonoran genera. 

 But a single subtropical bird is known, namely, Dendroica bry- 

 anti castaneiceps, and it is restricted to the mangrove lagoons. 



The presence of this subtropical bird in the narrow coast 

 lagoons is in complete accord with the vegetation of the coast 

 strip, which, as Mr. T. S. Brandegee tells us, is subtropical.* 

 This indicates the presence of a narrow coast belt similar to that 

 of southern Florida, but of less extent. It is possible that Basi- 

 linna xantusi is subtropical rather than Sonoran, but the details 

 of distribution of the genus are not well known. 



Among reptiles, about 25 peculiar species of snakes and lizards 

 are believed to be restricted to the peninsula, but no peculiar 

 genus is known. Three of the genera are tropical, and nine are 

 arid Lower Sonoran. 



In addition to the peculiar species and subspecies of the 

 peninsula, many characteristic arid Lower Sonoran forms of 

 mammals, birds, reptiles, insects, and plants abound. Among 

 the latter may be mentioned the highly distinctive Sonoran 

 desert brush, Larrea mexicana and Krameria parvijolia. 



Cope includes the whole peninsula in his Lower Cali/ornian 

 Region, but other writers restrict the peculiar fauna and flora to 

 the end of the peninsula south of the north foot of the mountains 

 between La Paz and Todos Santos. Bryant states : " There is 

 no more sharply denned faunal and floral area, that occurs to 



Brandegee, Proc. Calif. Acad. Sci., 2d ser., Ill, 1891, 110. 



