28 BULLETIN 18 8, UNITED STATES NATIONAL MUSEUM 



Yom (Mje Nam) : River tributary to the Mse Nan, watering the country between 



the eastern and western Phi Pan Nam Ranges ; N., C. 

 Yuam (Mffi Nam) : River tributary to the Salwin, on which Mae Sariang is 



situated ; W. 



LEADING FEATURES OF THE FISH FAUNA 



A fresh-water fish fauna of great variety and abundance has de- 

 veloped in Thailand as a result of peculiarly favorable physical and 

 biological conditions. The country extends through 16° of latitude 

 (5° to 21° N.) and 9° of longitude (97° to 106° E.) and has an exceed- 

 ingly diverse topography, comprising mountains, elevated plateaus, 

 flat alluvial plains, interior and coastwise swamps and marshes, and 

 mountainous and low islands, some of considerable size. About 70 

 percent of the surface is forest-clad, and mountains and lowlands 

 contain innumerable brooks, rivers, canals, and lakes, and the plains 

 have an intricate system of connecting and intersecting watercourses. 



Under the influence of the southwest monsoon, there exists over 

 most of the country a wet season of daily rainfall, which lasts from 

 April to November, with an average yearly precipitation of 1,600 

 mm. (63 inches). The amount of rainfall varies considerably in the 

 different parts of the countr}'^ and may average over 3,000 mm. (120 

 inches) in some places and only 900 mm. (36 inches) in others. The 

 rainy season is followed by a cool, dry, or winter season, under the 

 influence of the northeast monsoon, extending from November to 

 February, during which practically no rain falls. Between the cessa- 

 tion of the dry season and the beginning of the wet season there is a 

 short hot transition or summer season. 



The annual inundation of the vast central plain of Thailand and 

 of the various minor plains is an event of great importance in the 

 life of all the fishes. As the streams begin to rise and fill their beds, 

 together with the connecting canals, and the tributary ponds, lakes, 

 swamps, and marshes that had become reduced during the protracted 

 dry season, the fishes follow the flood waters, into the ricefields, into 

 the lakes, and into the swamps being converted into lakes ; and by the 

 time the inundation has reached its height the vast majority of the 

 free-swimming fishes have spawned. With the falling of the flood 

 waters, the adults move back into the river channels, leaving the 

 young to follow in accordance with their rate of growth and respec- 

 tive needs. The flood waters give protection to the 3'Oung, which in 

 general have a rapid growth and many attain maturity in one year. 



Outstanding among the fresh-water fishes are the Cyprinidae, which 

 are the most numerous as regards genera and species and the most 

 abundant as regards individuals. The present catalog recognizes 55 

 genera and 206 species. Some of the genera are peculiar to Siam, some 

 range into the contiguous countries, and into India, China, and the 



