sentences:  According to the original description of these groups, the "O. siamensis" species group is distinguished by having an emarginate caudal fin, and a lower lip notched medially with an entire or weakly laciniate posterior margin, while the "O. delacouri" species group is distinguished by having a lunate caudal fin and a lower lip without a median notch with prominent extensions along the posterior margin.It has been suggested that only the marginal morphology of lower lip can be employed to recognize the two species groups.There are currently 23 recognized species in this genus