26 



Laroides occidentalis, Bruch., J. f. O., 1855, 262. 



Larus argentatus var. ocddentalia, CouES, Key N. A. Birds, 1872, 312; Birds Northwest, 

 1874, 626. 



Locality : Lower California. In perfect breeding-plumage, well illus- 

 trating this form in the strength of the bill, slaty-grayish shade of the 

 mantle, &c. 



LARUS (BLASIPUS) HEERMANNI, Cass. [Nos. 70665, 70666, 70667]. 



Larus heermanni, Cass., Proc. Acad. Nat. Sci. Phila., vi, 1852, 187; Illns., 1853, 28, pi. 5. 

 Larus (Blasipus) heermanni, ScL. & Salv., Proc. Zool. Soc. London, 1871, 574. — Coles, 



Birds Northwest, 1874, 641. 

 Blasipus heermanni, Bp., Consp. Av., ii, 1856, 211. 

 Adelarus heermanni, Bruch., J. f. O., 1853, 107 ; 1855, 279. 

 Larns belcheri, Schl., Mas. P.-B., Lari, 9 (in part). 

 Larus {Balsipus) belcheri, CouES, Key N. A. Birds, 1872, 314 (in part). 



Locality: Isla Baza, Gulf of California. Isla Raza is the particular 

 breeding-place of these gulls in the gulf. It is a small, low island, about 

 three-quarters of a mile long and half a mile wide. At the time of our 

 visit (April), immense numbers of the birds were congregated there, 

 preparatory to laying their eggs, which, however, they had not begun to 

 deposit. We may safely say, without exaggeration, that there was a bird 

 on every square foot of the ground, and others were continually hovering 

 about overhead. Their incessant noise deadened all other sounds, and so 

 intent were they in their all-absorbing duties of reproduction, that they 

 seemed entirely unconscious of our presence amongst them. The forma- 

 tion of the island is a black volcanic rock, entirely destitute of vegetation. 

 Through the long series of years during which these birds have made it a 

 breeding-place, there has been going on a chemical reaction between the 

 acids of their excrement and the bases of the rock, which has resulted in 

 the formation of a new substance, composed largely of a tri-basic phos- 

 phate. This now forms (or did form) a thick layer, covering the whole 

 surface of the island. On breaking open the bowlders, a sharp line of 

 demarkation can be seen extending into the body of the rock showing 

 the depth of the chemical reaction. The altered rock being a softer 

 material than the original is easily pulverized and worn off by the con- 

 stant attrition of the birds' feet during their breeding-season. In this 

 way, the inequalities of the surface of the rocky islet have been smoothed 

 over. A copapany has possession of the island, and is gathering the 

 guano. Ten thousand tons have been removed, and it is calculated 

 that six times that quantity still remains to be gathered (1875). 



In perfect breeding-dress. The eyelids are red, like the bill. 



