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region is characterized by a peculiar structure of tbc rostral plate, which 

 is produced either anteriorly or laterally to an unusual degree ; two of 

 the genera {Pliimothyra and ChUomcnlscm), common to the Lower Cali- 

 fornian region, present the same peculiarity. 



This region is the headquarters of the rattlesnakes, there being no 

 less than nine species found in it, of which six are peculiar. It also 

 possesses a majority of the species of horned toads {Phnjnosoma); only 

 four of the North American species being unknown there. The Testu- 

 dinate fauna is very poor, jmssessing a few species of Nearctic charac- 

 ter, and three Cinosterna, two of them of Mexican type. 



The Batrachian fauna exhibits but one genus of Urodda, but several 

 of the Amim. Appropriately to its arid character, there is but one Rana, 

 but six species of toad (Biifo), this being the headquarters of that genus 

 in the Kegnum Nearcticum. .The eastern genus ScapMopus appears here, 

 instead of the Spea of the other western regions. There is one species 

 of tree-frog. 



Two species of turtles of the Ciuosternidae have been found. The 

 fresh-water fish-fauna is very poor, and but little known. In the Colo- 

 rado River proper, the Salmonidac and Cottidae appear to be wanting, 

 leaving only Cyprlnidae and Catostomidac. A strongly-marked division 

 of the former, the Plagojjterinae, which embraces three genera, is mainly 

 restricted to the Colorado River drainage, and is the most striking feature 

 of the fish-fauna of the Sonoran region. 



The Lower CaUfornian region much more nearly resembles the youoraii 

 than the Pacific region. It possesses, however, many peculiar species 

 of birds and reptiles.^ Seines appear to be wanting, but other lizards 

 abound. The following genera of reptiles have been found here, which 

 do not occur in any other region of Nearctica : 



Lizards. 



Verticaria. 



DiplodacUjlus. 



Cyclura, 





Snakes. 

 Lichaniira. 



These, except the last, have been found in Mexico or South America. 

 It shares with the Sonoran only, the following : 



