MAMMALS OF THE TRES MARIAS ISLANDS. 



By E. W. NELSON. 



Mammals are not numerous either in species or individuals upon the 

 Tres Marias. So far as known, they number but eleven species, of 

 which seven are peculiar to the islands; one is introduced, and the 

 other three are widely ranging bats. A sea lion and two species of 

 porpoise were found near the shores, and whales were reported to 

 occur during certain seasons. As with the birds, one of the most 

 unaccountable features of the mammal fauna is the absence of a num- 

 ber of species that are common on the adjacent mainland. Considering 

 the primitive condition of the islands, it is difficult to explain the 

 presence of field mice, the pigmy opossum, rabbit, and raccoon, while 

 the large gray opossum, nasua, skunk, fox, coyote, deer, peccary, 

 squirrel, and various small rodents of the adjacent mainland remain 

 unrepresented. The Tres Marias mouse was rather common above 200 

 feet on all of the larger islands; the rabbit was very numerous near 

 the north end of Maria Madre, on San Juanito, and in some places on 

 Maria Magdalena, and two species of bats were abundant in caves on 

 Maria Madre. Aside from these species, mammals were uncommon 

 and difficult to find. One cause of their general scarcity may be the 

 very limited supply, of permanent fresh water, and the absence of small 

 species from a broad belt near the shore was easily accounted for by 

 the abundance of carnivorous crabs. 



The mammals obtained by our party have been identified by Dr. C. 

 Hart Merriam, who has described the new forms and given critical 

 notes on other species. 1 Of the land mammals taken, five were new 

 and two, Lepus graysoni and Rhogeessa parvula, had been previously 

 described. We failed to secure two species of bats (Myotis nigricans 

 and Lasiurus borealis mexicanus) which were taken by Mr. Forrer. Not- 

 withstanding the fact that collections were made in several branches 

 of natural history, I feel confident that representatives of all the resi- 

 dent land mammals were secured, but it is quite possible that future 

 work may add other bats to the present list. 



ANNOTATED LIST OF SPECIES. 



Marmosa insularis Merriam. Tres Marias Pigmy Opossum. 



Marmosa insularis Merriam. Proc. Biol. Soc., Washington, XII, pp. 14-15, Jan. 



27, 1898. Type from Maria Madre Island. 



These pretty little opossums were not found except in the high inte- 

 rior of Maria Madre, between 1,200 and 1,800 feet above sea level, where 



' Proc. Biol. Soc. Washington, XII, pp. 13-19, 1898. 



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