Odocoileus virginianus (Boddaert): White-tailed Deer.—A weathered Recent fragment of a mandible (89048) and part of a femur (89049) of this species were found near Camp 1 on July 7, and a metapodal was picked up in the dunes at Camp 2 on July 9. This species has not been reported previously on the barrier island of Tamaulipas and it probably no longer occurs there, for we saw no tracks or other signs of it. Hall (1951) did not find it at Eighth Pass.
Our specimens probably pertain to O. v. texanus but are possibly of O. v. veraecrucis, which has been reported from Soto la Marina (Goldman and Kellogg, 1940:89).
The only species of mammal known from the barrier island of Tamaulipas that we did not find is the Hispid Cotton Rat (Sigmodon hispidus). Two specimens of this species trapped near Eighth Pass in March, 1950, formed the basis for the description of S. h. solus (Hall, 1951:42), a subspecies known only from the type locality.
Discussion
The known vertebrate fauna of the barrier island of Tamaulipas consists of one species of tortoise, two species of lizards, at least one (unidentified) species of snake, 49 species of birds (48 recorded by us and the Semipalmated Sandpiper), and 12 species of mammals. This is clearly a depauperate fauna, such as is characteristic of islands generally, and indicates that the peninsular nature of the northern part of the barrier island is of relatively small consequence in determining presence or absence of species. It is likely that the restricted environmental spectrum is much more important in this regard than is the fact of semi-isolation.
Of the 49 species of birds, 10 are known to breed on the island and an additional 21 are suspected of breeding either on the island or on small islets in the adjacent Laguna Madre de Tamaulipas. Eleven species occur on the island as nonbreeding summer residents, about which we will have more to say below. Four species have been recorded on the island in summer but breed elsewhere, that is to say, they only wander over the island (Man-o'-war Bird, Turkey Vulture, etc.). Two species are known only as migrants, and the status of one, the Sora Rail, is uncertain. The number of migrant species doubtless will be greatly increased by field work at those times when birds migrate.
The avifauna is not depauperate owing to the exclusion of any one of the three major zoogeographic stocks thought to be important in the development of the present North American avifauna (Mayr, 1946). If we examine the breeding passerine birds of the barrier island and the breeding passerine assemblage at the same latitude in lowland Sonora (Mayr, loc. cit.) as to their ultimate evolutionary sources, we find that for both places somewhat more than half the birds have developed from indigenous, North American stocks, about one-third have been derived from South American stocks, and one-fifth to one-eighth are from Eurasian stocks. It is most unlikely that such close correspondence in relative composition of the two avifaunas would occur by chance. Thus, we can only conclude that each of the historical avian stocks is proportionately restricted in numbers on the barrier island.
Faunistically, the barrier island resembles Padre and Mustang islands and the adjacent mainland of Tamaulipas and southern Texas, reflecting the relative uniformity of environment in this region. It is apparent that there is a faunal "break" or region of transition in the vicinity of Tampico, in extreme southeastern Tamaulipas. On the coastal plain, many tropical species and subspecies occurring in Veracruz are found north to Tampico but fail to extend farther northward to the barrier island of northeastern Tamaulipas. Axtell and Wasserman (1953:4-5), have already commented on this situation, mentioning a number of snakes and lizards that have differentiated subspecifically on opposing sides of the Tampican region. They also note that large numbers of the lowland Neotropical floral and faunal elements reach their northern limits of distribution within the zone of transition around Tampico, and, also, many Nearctic elements find their southern distributional limits there.
Our small samples of birds and reptiles from the island show no detectable morphological differentiation from adjacent populations. However, several of the mammals are moderately-well differentiated, but the patterns and degrees of geographic variation are such that we can only speculate on the historical derivation of the insular populations. Lepus californicus curti is presently known only from the barrier island of Tamaulipas, but Hall (1951:43) has suggested that it may also occur on the adjacent mainland. A resemblance between individuals of this subspecies and specimens of L. c. merriami from Padre Island in smallness of the tympanic bullae is regarded, probably correctly, by Hall (1951:44) as independent development—that is, parallel adaptation to similar environmental conditions reaching fullest expression on the barrier island of Tamaulipas. As is also true with Geomys personatus and Neotoma micropus, the barrier island population of Lepus californicus shows relationships with animals from Texas and northern Tamaulipas (L. c. merriami) and no connection with (resemblance to) animals from the south (L. c. altamirae, known only from the type locality at Altamira, near Tampico).