74 



Records of the Indian Museum. 



[Vol.. XIX, 



cresced at their posterior ends, as seen at present in Tagelus, and 

 the fusion of the two mantle lobes gradually extended to the ante- 

 rior end as the various genera originated from them. The fam. 

 Psammobiidae, closely related to Solecurtinae, might have arisen 

 from the same ancestral forms after the differenciation of a cruci- 

 form muscle in the fused ventral margins of the mantle lobes. The 

 evolution of the muscle can at present be followed in the different 

 genera of the subfam. Solecurtinae, the primitive condition as a 

 simple transverse band being found in Tagelus. 



Returning to the subfam. Soleninae, we find the genera 

 Siliqiia and Ceratosolen differing from the others in having simple 

 non-plicate gills with irregular interlamellar junctions ; but the 

 l^lication of the gills has no taxonomic value more than as a sub- 

 generic character as shown by Ridewood (26, pp. 161-2). 



The relation between the genera of the three subfamilies may 

 be thus provisionally represented : — 



NoucLCuZvrvina&. 



Frirnvtive^ forrrv. 



