FRESH-WATER FISHES OF SIAM, OR THAILAND 501 



In speculations as to how and why animals have left the stable, dependable 

 ocean in order to take up a precarious existence in highly variable land habitats, 

 various factors have been stressed. Doubtless enemies, desirable foods, lack of 

 oxygen, reproduction, and other things have been more or less effective as con- 

 tributing causes to such migrations. The writer * * * ij^s suggested that an 

 important factor in the adjustments of animals to land life is the avoidance of 

 interspecific competition. Many species of animals along the shores of oceans are 

 arranged in definite zones and thus avoid competition. * * * Incidentally, 

 some of them acquire such qualities that they can live on land. The species of 

 beach gobies at Paknam are not arranged in definite zones but compete very little 

 with one another because they are somewhat specialized in their food habits. 

 They are thus together but not segregated. Wherever there is vacant territory 

 or habitat in nature there is a chance for a new species. Safety, unconsumed 

 resources, lack of competition, desirable breeding places, and other desiderata 

 make the enduring of new hardships expedient. Thus land animals have evolved 

 because certain types could avoid old competitions by acquiring new ranges of 

 adjustment so that they could live in vacant habitats. 



In Thailand as in various other tropical or subtropical countries 

 the size, abundance, and food value of the gobies make them of con- 

 siderable economic importance. In the Philippines and elsewhere the 

 newly hatched young on their way from the sea back to the stream 

 from which their parents migrated are taken in immense quantities 

 and are the basis of an important industry. 



Eather than attempt to separate the species living in fresh and in 

 brackish waters from those in brackish and salt waters, it has been 

 thought advisable in the case of the present order to depart from the 

 general treatment observed in the catalog and to enumerate all the 

 known local species. These are included in the following genera and 

 subgenera : 



la. Form not eellike; dorsal fins separate (rarely united at base). 

 2a. Eyes not stalked ; no free movable lower eyelid ; teeth in one or several to 

 many rows in each jaw; pectoral fins with no noteworthy muscular de- 

 velopment at their base. 



3a. Ventral fins close together but not joined Eleotridae 



3&. Ventral fins united and having across their base a membrane or frenum 

 which forms a sucking disk, or basal membrane partly or wholly 



deficient Gobiidae 



4a. Body more or less elongate ; head and body compressed or depressed ; 

 head scaleless or partly scaled; body scaleless, partly scaled, or 

 fully scaled; gill openings restricted to sides or extending more or 

 less forward ; ventral fins united into a disk or joined basally with 

 or without a connecting membrane. 

 5a. Teeth in several rows in each jaw (in one row in upper jaw 



in Oxyurichthys Gobiinae 



56. Teeth in a single row in each jaw Sicydiinae 



4b. Body elliptical and strongly compressed ; head and body entirely scale- 

 less or body covered with large ctenoid scales ; gill openings re- 

 stricted to sides ; ventral fins short, united into a disk or tube. 



Gobiodontinae 

 590087 — 45 33 



