GENERAL DESCRIPTION OF THE TRES MARIAS ISLANDS. 11 



volcanic origin and exhibits similar evidence of having once been a 

 much larger island which is now sinking. Apparently it consists mainly 

 of the remains of an old volcano, and a small crater still occupies the 

 center of the island. Although no craters were seen on the Tres 

 Marias, yet there are lavas and other volcanic rocks on all the islands, 

 but a large part of the formation is made up of other rocks elevated by 

 the volcanic uplift. 



FAUNA. 



The Tres Marias, like the adjacent coast, lie within the Arid Tropical 

 life zone. The evidence furnished by the fauna of the former connec- 

 tion of the Tres Marias with the mainland is as follows : Six species 

 of land shells were obtained, which, according to Dr. William H. Ball, 

 are widely distributed on the mainland. These species are Polygyra 

 ventrosula Pfr., Orthalicus undatus Brug., Orthalicus undatm melano- 



cheilus Val., Lamellaxis ?, Opeas subula Pfr., and Glandina turris, 



Pfr. A fresh- water fish taken on Maria Magdalena and Maria Cleofa 

 has been identified by Prof. B. W. Evermann as Agonostomus nasutus 

 Giinther, a common species on the mainland. In fresh-water pools on 

 Maria Magdalena two or three individuals of another small fish were 

 seen, which were very similiar to common mainland species of Aicaous, 

 and undoubtedly belong to this or a closely allied genus. Six of the 

 seven species of lizards inhabit the mainland, and only one is peculiar 

 to the islands; the mud turtle and crocodile are also found on the 

 mainland, as are the eight species of snakes. Concerning the reptilian 

 fauna Dr. Stejneger remarks: "Thus most of the species are common 

 on the opposite mainland and generally distributed over tropical Mexico 

 and Central America. Then again it seems as if the species are prac- 

 tically identical on all the islands of the group. This would indicate 

 a comparatively recent severance of these islands from each other, as 

 well as from the opposite mainland of Mexico." 



The birds and mammals seem to have been more susceptible to modi- 

 fying influences than other forms of life. Thirty-six species of resident 

 land birds were found on the group, of which twelve are identical with 

 those on the mainland, and twenty-four can be distinguished specifically 

 or subspecifically. We found ten species of indigenous mammals, seven 

 of which, acccording to Dr. Merriam, are peculiar to the islands, but 

 closely related to species living on the mainland. 



ANIMALS PECULIAR TO THE TRES MARIAS. 



So far as known, the following species and subspecies (with the excep- 

 tion of Compsothlypis insularis) are peculiar to the islands: 



Marmosa insularis Merriam. Procyon lotor insularis Merriam. 



Oryzomys nelsoni Merriain. Bhoyeessa parvula H. Allen. 



Peromyscus madrensis Morriam. Glossophaga mutica Merriam. 

 Lepus grayeoni Allen. 



