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). In color and cranial proportions, _Dipodomys ordii parvabullatus_ of the barrier island is closer to _D. o. compactus_ of Padre Island than to _D. o. sennetti_ of southern Texas and the Tamaulipan mainland. But, _D. o. parvabullatus_ resembles _D. o. sennetti_ in external measurements (Hall, 1951:39). Possibly _D. o. parvabullatus_ and _D. o. compactus_ are phylogentically closer to one another than is either to _D. o. sennetti_. It is also possible that each evolved independently from a mainland stock represented today by _D. o. sennetti_; the resemblance of the two insular populations would thus be a matter of convergence in response to like environmental conditions.