254 GILBliRT 0. BOURNE. 



mc that tlie specimens he collected were uncovered not only 

 at low tides, but also at liigli neap tides, so that they re- 

 mained exposed for days together to the rays of a tropical 

 sun, but nevertheless always remained moist and fresh. The 

 power of resisting desiccation for so long a time is unusual 

 among lamellibranchiate molluscs, and indicates some struc- 

 tural adaptations enabling the animal to survive in such a 

 habitat. I shall show in the course of this paper that the 

 thickened mantle lobes and the presence of certain irregular 

 passages and channels connected with the mantle cavity may 

 be regarded as modifications for the retention of a sufficient 

 supply of moisture, but in the essential features of its 

 anatomy Enigma is very similar to Anomia ephippium. 



Enigma, like other Anomiacea, is firmly fixed by a stout 

 byssus passing through a large sinus or circular perforation 

 in the right valve of the shell. The specimens given me by 

 Mr. Shelford have been carefully preserved for histological 

 purposes, and the right valve has in every case been wholly 

 or partially removed. I am therefore unable to give an exact 

 account of it, but from the fragments remaining in several 

 specimens, I can confirm the descriptions given in coucho- 

 logical works, namely, that the upper extremities of the sinus 

 through which the byssus passes curve round above the 

 byssus and meet one another above the ligament, but do not 

 fuse together as in adult specimens of Anomia epliip- 

 p i u m . 



The left valve varies very much in size and shape, and its 

 form seems to be moulded to some extent upon the substra- 

 tum to which the animal is attached. It is generally more 

 or less elongate-oval in shape, of a dark purple colour, nacre- 

 ous and iridescent internally. It is very thin and more or 

 less translucent, especially when wet, this latter character 

 being apparently of some importance to the economy of the 

 animal, as will appear later on. 



There are certain points of difference between the left 

 valve of Enigma and that of Anomia which require some 

 little explanation. Tn the latter genus the left valve is more 



