SOWEEBY : NEW TRIDACKA FKOM THE PHILIPPINES. 211 



lunula concave depressa ; hiatus parvus ; umbones incurvati, contigui, 

 ante medium siti. Long. 16, lat. 12, crass. 10 cm. 



This species is remarkable for its smoothness and obesity, the 

 smallness of the byssal opening, very low ribs, and peculiar form. 

 In all the hitherto known species the umbones are situated either in 

 the centre or towards the posterior end of the dorsal line, whereas in 

 T. ohesa they are nearer to the anterior. 



Of the three specimens, I have chosen the one of medium size as the 

 type, since it is the most characteristic and regular in form. The 

 largest shell (21 cm. in length) is similar in essential characters, but 

 somewhat distorted; whilst the smallest . (14 cm. long) differs but 

 little from the type. 



