Crocethia alba (Pallas): Sanderling.—This species was relatively common at Sisal where several flocks of 30 to 40 birds were seen. One specimen (40444) shows an unusual pattern of molt; replacement of outer primaries has preceded that of the inner.
Specimens (4): ♂, 40445, testis 3 × 1 mm., 57.5 gm., heavy fat (August 9), ♀, 40444, ovary granular 6 × 3 mm., 56.7 gm., molting (August 11), ♂, 40443, 35.5 gm. (August 11), and ♀, 40446, imm., 54.1 gm. (August 11), 6½-13 km. WSW Sisal. Mallophaga: Lunaceps holophaeus actophilus, August 11; Carduiceps zonarius, August 11.
Ereunetes pusillus (Linnaeus): Semipalmated Sandpiper.—The only individual seen was taken from a mixed flock of Sanderlings and Wilson Plovers.
Specimen (1): ♂, 40447, testis minute, 24.4 gm. (August 11), 8 km. WSW Sisal.
Himantopus mexicanus (Müller): Black-necked Stilt.—A lone individual was sighted on Isla Mujeres on August 2. A pair was on the mudflats among mangroves at Sisal, of which the male was taken on August 7. Although Paynter (1955a) indicated that this species probably breeds on the peninsula, there are no definite breeding records. Our specimen had only slightly enlarged testes, but its close association with a female suggested that the pair may have been mated.
Specimen (1): ♂, 40448, testis 7 × 3 mm., 189.4 gm., little fat (August 7), 5 mi. SW Sisal.
Larus atricilla Linnaeus: Laughing Gull.—This species was seen regularly along the coasts of the peninsula.
Specimen (1): ♀, 40449, ovary 7 mm. (inactive), 253 gm. (August 7), 4-3/10 mi. W Sisal. Mallophaga: Quadraceps sp.
Sterna albifrons antillarum (Lesson): Least Tern.—A flock of about 20 individuals was seen on Isla Mujeres; two individuals were shot and appear to be the first specimens from the island.
Specimens (2): ♀, 40451, ovary slightly enlarged, 49.0 gm. (August 2); sex? 40450, 41.8 gm. (August 2), Isla Mujeres.