The glochidia of S. khwaenoiensis are classified with in the hook less category as are the other twelve Thai Unionoid species previously reported. The general shell structure is similar to that of Physunio superbus, Pseudondon cambodiensis and Uniandra contradens within the second group (unequivalve, unequilateral). However, the external and internal shell surface structures have unique porous characteristics and the shell height value of S. khwaenoiensis is larger than all the other reported Thai species. These findings will provide useful informative characters to assist in S. khwaenoiensis identification and further classification of (Thai) freshwater unionid mussels.