IN order to make a name available for use in papers by the junior author and others, we anticipate the appearance of our long unfinished revision of the xiphophorin fishes by herein naming and diagnosing the pigmy swordtail: Xiphophorus pygmaeus, new species TYPES.-The holotype is an adult male 25 mm. in standard length (P1. 1, fig. 1), collected by Myron Gordon and Salvador Coronado in Rio Axtla of the Rio Panuco system, at Axtla, San Luis Potosi, Mexico, on April 14, 1939; University of Michigan Museum of Zoology, Cat. No. 124365. With the holotype there were seined 173 paratypes, 14 to 32 mm. long, including 16 completely transformed males (UMMZ, 124366). A transforming male (UMMZ, 108601), 16 mm. long, was collected by Gordon, Whetzel and Ross at the same locality, on April 20, 1932. Others were collected by an expedition of the New York Zoological Society in Rio Axtla at Axtla and in an arroyo between Rio Axtla and Rio Moctezuma, both on March 25, 1940. The largest specimen (UMMZ, 124340), a somewhat aberrant adult female 40 mm. long, the only one not designated as a paratype, was taken by Gordon and party in Rio Matalpa, at Matalpa, San Luis Potosi, 13 miles north of Tamazunchale, on April 14, 1939. Live stock from Axtla, therefore closely related to the type series, has been maintained by the junior author for use in genetic research. The habitat of the species is described by Gordon (1940:173). DIAGNOSIS AND RELATIONSHIPS.-On the basis of gonopodial and other characters this species clearly enters the tribe Xiphophorini Hubbs (1924: 10, P1. 3, fig. 4). This group, comprising the genera Platypoecilus Gunther and Xiphophorus Heckel, had already been recognized, though not named, by Regan (1913: 980, 1004-1005, figs. 172 D-E). The gonopodium of X. pygmaeus, unlike that of X. montezumae Jordan and Snyder, does not approach that of Platypoecilus: the recurved terminal segment of ray 5a is fully developed and the distal serrae on ray 4p are short. However, the swordlike prolongation of the lower caudal rays of the adult males, heretofore thought to be diagnostic of Xiphophorus, is but weakly developed (P1. 1, fig. 1), or rudimentary-often less produced than in Platypoecilus xiphidium Hubbs and Gordon. The number of dorsal rays (Table 1) averages even lower than in Platypoecilus variatus, but the distinction from other species of Xiphophorus, especially from X. montezumae, is also only an average one. Therefore the number of dorsal rays can no longer be used as a generic feature. Because the other distinctions between Platypoecilus and Xiphophorus tend to break down (the gonopodial characters in X. montezumae and the caudal sword character in P. xiphidium and X. pygmaeus), and since the